Crystallisation of 5-X-aspirin (X = Cl, Br) by ambient evaporation from organic solvents commonly yields a mixture of polymorphs I and II for X = Cl, but only polymorph I for X = Br. Addition of 5-X-aspirin anhydride (5–20 mol %) leads to sole production of polymorph II for X = Cl, and new production of polymorph II for X = Br. Slurry experiments show that polymorph I transforms to polymorph II for X = Cl, while polymorph II transforms to polymorph I for Br. Thus, addition of the anhydride during crystallisation apparently accelerates the I → II transformation for X = Cl, and inhibits the II → I transformation for X = Br. 5-X-Aspirin anhydride can be produced as a by-product during heating of 5-X-aspirin in organic solvents, or during synthesis from 5-X-salicylic acid, and the duration of the heating step in such a synthesis can therefore change the polymorphic form of the synthesised product. The 5-X-aspirin anhydrides (X = Cl, Br, Me) are also polymorphic. Crystallisation of the pure compound from organic solvents yields only polymorph I, while polymorph II can be obtained by heating in ethanol or acetone. For X = Cl or Br, the heating step produces significant quantities of 5-X-aspirinin situ, while for X = Me, it is necessary also to add 5-X-aspirin (5–20 mol %) to obtain polymorph II.