A molecular modelling procedure for the prediction of interfacial energies between polymorphs and solvents is under development. The first step in such a procedure must be to determine and investigate the structure of the possible morphologically important crystal surfaces. This is done for two of the experimentally readily obtained polymorphs beta-HMX and alpha-HMX (cyclotetramethylene tetramine) by means of a periodic bond chain analysis. (C) 2002 Elsevier Science B.V. All rights reserved.