diff --git a/gateway/events.go b/gateway/events.go
index f35032c..472a4b6 100644
--- a/gateway/events.go
+++ b/gateway/events.go
@@ -1,10 +1,13 @@
 package gateway
 
 import (
+	"encoding/json"
+	"errors"
 	"strconv"
 	"strings"
 
 	"github.com/diamondburned/arikawa/v3/discord"
+	"github.com/diamondburned/arikawa/v3/utils/json/option"
 	"github.com/diamondburned/arikawa/v3/utils/ws"
 )
 
@@ -69,19 +72,52 @@ type ResumeCommand struct {
 // https://discord.com/developers/docs/topics/gateway#connecting-and-resuming
 type InvalidSessionEvent bool
 
-// RequestGuildMembersCommand is a command for Op 8.
+// RequestGuildMembersCommand is a command for Op 8. Either UserIDs or (Query
+// and Limit) must be filled.
 type RequestGuildMembersCommand struct {
-	// GuildIDs contains the ids of the guilds to request data from. Multiple
+	// GuildIDs contains the IDs of the guilds to request data from. Multiple
 	// guilds can only be requested when using user accounts.
-	GuildIDs []discord.GuildID `json:"guild_id"`
-	UserIDs  []discord.UserID  `json:"user_ids,omitempty"`
+	GuildIDs []discord.GuildID `json:"guild_ids"`
+
+	// UserIDs contains the IDs of the users to request data for. If this is
+	// filled, then the Query field must be empty.
+	UserIDs []discord.UserID `json:"user_ids,omitempty"`
+
+	// Query is a string to search for matching usernames. If this is filled,
+	// then the UserIDs field must be empty. If Query is empty, then all members
+	// are filled for bots.
+	Query option.String `json:"query,omitempty"`
+	// Limit is used to specify the maximum number of members to send back when
+	// Query is used.
+	Limit uint `json:"limit,omitempty"`
 
-	Query     string `json:"query,omitempty"`
-	Limit     uint   `json:"limit,omitempty"`
 	Presences bool   `json:"presences"`
 	Nonce     string `json:"nonce,omitempty"`
 }
 
+// MarshalJSON marshals a RequestGuildMembersCommand.
+func (c *RequestGuildMembersCommand) MarshalJSON() ([]byte, error) {
+	type raw RequestGuildMembersCommand
+
+	if c.UserIDs != nil && c.Query != nil {
+		return nil, errors.New("neither UserIDs nor Query can be filled")
+	}
+
+	var marshaling interface{} = (*raw)(c)
+	if c.Query != nil {
+		// Force the Limit field to be present if Query is present.
+		marshaling = struct {
+			*raw
+			Limit uint `json:"limit"`
+		}{
+			raw:   (*raw)(c),
+			Limit: c.Limit,
+		}
+	}
+
+	return json.Marshal(marshaling)
+}
+
 // UpdateVoiceStateCommand is a command for Op 4.
 type UpdateVoiceStateCommand struct {
 	GuildID   discord.GuildID   `json:"guild_id"`
diff --git a/gateway/events_test.go b/gateway/events_test.go
new file mode 100644
index 0000000..1217b53
--- /dev/null
+++ b/gateway/events_test.go
@@ -0,0 +1,89 @@
+package gateway
+
+import (
+	"context"
+	"encoding/json"
+	"reflect"
+	"strings"
+	"testing"
+
+	"github.com/diamondburned/arikawa/v3/discord"
+	"github.com/diamondburned/arikawa/v3/utils/json/option"
+	"github.com/diamondburned/arikawa/v3/utils/ws"
+)
+
+func TestRequestGuildMembersCommand(t *testing.T) {
+	assert := func(cmd Event, data map[string]interface{}) {
+		cmdBytes, err := json.Marshal(cmd)
+		if err != nil {
+			t.Fatal("failed to marshal command:", err)
+		}
+
+		var cmdMap map[string]interface{}
+		if err := json.Unmarshal(cmdBytes, &cmdMap); err != nil {
+			t.Fatal("failed to unmarshal command:", err)
+		}
+
+		if !reflect.DeepEqual(cmdMap, data) {
+			t.Fatalf("mismatched command, got %#v", cmdMap)
+		}
+	}
+
+	t.Run("userIDs", func(t *testing.T) {
+		cmd := RequestGuildMembersCommand{
+			GuildIDs: []discord.GuildID{123},
+			UserIDs:  []discord.UserID{456},
+		}
+
+		assert(&cmd, map[string]interface{}{
+			"guild_ids": []interface{}{"123"},
+			"user_ids":  []interface{}{"456"},
+			"presences": false,
+		})
+	})
+
+	t.Run("query_empty", func(t *testing.T) {
+		cmd := RequestGuildMembersCommand{
+			GuildIDs: []discord.GuildID{123},
+			Query:    option.NewString(""),
+		}
+
+		assert(&cmd, map[string]interface{}{
+			"guild_ids": []interface{}{"123"},
+			"query":     "",
+			"limit":     float64(0),
+			"presences": false,
+		})
+	})
+
+	t.Run("query_nonempty", func(t *testing.T) {
+		cmd := RequestGuildMembersCommand{
+			GuildIDs: []discord.GuildID{123},
+			Query:    option.NewString("abc"),
+		}
+
+		assert(&cmd, map[string]interface{}{
+			"guild_ids": []interface{}{"123"},
+			"query":     "abc",
+			"limit":     float64(0),
+			"presences": false,
+		})
+	})
+
+	t.Run("both", func(t *testing.T) {
+		cmd := RequestGuildMembersCommand{
+			GuildIDs: []discord.GuildID{123},
+			UserIDs:  []discord.UserID{456},
+			Query:    option.NewString("abc"),
+		}
+
+		// Gateway should never be touched when Marshal fails, so we can just
+		// create a zero-value.
+		var gateway ws.Gateway
+
+		err := gateway.Send(context.Background(), &cmd)
+		if err == nil || !strings.Contains(err.Error(), "neither UserIDs nor Query can be filled") {
+			t.Fatal("unexpected error:", err)
+		}
+	})
+}