Two years after this case was removed to federal court on diversity grounds, the parties discovered that one of the properties at issue in the case was partly owned by a trust of which one trustee was a non-diverse California resident. The plaintiff moved to join the non-diverse trustee as a defendant. The district court granted the joinder, severed the claim dealing with the property partly owned by the trust, and remanded that claim to state court under 28 U.S.C. 1447(e). This decision holds that section 1447(d) bars the Court of Appeal from reviewing the remand order or the order joining the nondiverse defendant either on appeal or by writ of mandate. Section 1447(d) bars appeals from remand order under either 1447(c) or 1447(e). While the decision to allow joinder of a nondiverse defendant is discretionary, remand after such a joinder is allowed is mandatory. While an appellate court may review a remand order for the limited purpose of determining whether it is colorably based on lack of subject-matter jurisdiction, a remand due to joinder of a non-diverse defendant is plainly based on lack of subject matter jurisdiction. The appellate court cannot go further to decide whether the remand was legally correct. Nor can the joinder ruling be reviewed as a prior separable order since the joinder ruling did not conclusively determine any claim. The state courts can undo the joinder if they find it improper.