diff --git a/js/common-vsm/model/LauncherMechanism.ts b/js/common-vsm/model/LauncherMechanism.ts
index 4a1b4ce..0012dfa 100644
--- a/js/common-vsm/model/LauncherMechanism.ts
+++ b/js/common-vsm/model/LauncherMechanism.ts
@@ -60,7 +60,7 @@ export default class LauncherMechanism extends PhetioObject {
    * at startup and exist for the lifetime of the simulation, it implements 'Reference type serialization', as described in the
    * Serialization section of https://github.com/phetsims/phet-io/blob/main/doc/phet-io-instrumentation-technical-guide.md#serialization
    */
-  public static LauncherMechanismIO = new IOType<LauncherMechanism>( 'LauncherMechanismIO', {
+  public static readonly LauncherMechanismIO = new IOType<LauncherMechanism>( 'LauncherMechanismIO', {
     valueType: LauncherMechanism,
     documentation: 'Mechanism used to launch the projectile',
     supertype: ReferenceIO( IOType.ObjectIO )
diff --git a/js/common/model/Field.ts b/js/common/model/Field.ts
index b36b341..6079123 100644
--- a/js/common/model/Field.ts
+++ b/js/common/model/Field.ts
@@ -371,7 +371,7 @@ export default abstract class Field extends PhetioObject {
    * In this simulation, the data and statistical measures are computed solely based on the landedProjectiles, so the
    * serialization separates the projectiles into the airborne vs landed projectiles.
    */
-  public static FieldIO = new IOType<Field>( 'FieldIO', {
+  public static readonly FieldIO = new IOType<Field>( 'FieldIO', {
     valueType: Field,
     documentation: 'A field in the Projectile Data Lab. This contains the state for the projectiles, separated into airborne and landed projectiles.',
     defaultDeserializationMethod: 'applyState',
diff --git a/js/common/model/Projectile.ts b/js/common/model/Projectile.ts
index 18f8ce9..5c0cb8b 100644
--- a/js/common/model/Projectile.ts
+++ b/js/common/model/Projectile.ts
@@ -140,7 +140,7 @@ export default class Projectile {
    * calls ProjectileIO.toStateObject to serialize the Projectile instances using value-based serialization (and vice-
    * versa for fromStateObject).
    */
-  public static ProjectileIO = new IOType<Projectile, ProjectileStateObject>( 'ProjectileIO', {
+  public static readonly ProjectileIO = new IOType<Projectile, ProjectileStateObject>( 'ProjectileIO', {
     valueType: Projectile,
     stateSchema: {
       screenIdentifier: StringUnionIO( ScreenIdentifierValues ),
diff --git a/js/common/model/ProjectileType.ts b/js/common/model/ProjectileType.ts
index 15f32de..b4c6786 100644
--- a/js/common/model/ProjectileType.ts
+++ b/js/common/model/ProjectileType.ts
@@ -41,7 +41,7 @@ export default class ProjectileType extends PhetioObject {
    * at startup and exist for the lifetime of the simulation, it implements 'Reference type serialization', as described in the
    * Serialization section of https://github.com/phetsims/phet-io/blob/main/doc/phet-io-instrumentation-technical-guide.md#serialization
    */
-  public static ProjectileTypeIO = new IOType<ProjectileType>( 'ProjectileTypeIO', {
+  public static readonly ProjectileTypeIO = new IOType<ProjectileType>( 'ProjectileTypeIO', {
     valueType: ProjectileType,
     documentation: 'The type of object being launched.',
     supertype: ReferenceIO( IOType.ObjectIO )