The variation in physical properties of crystals grown in the presence of additives or impurities have previously been attributed to lattice disorder developed during crystallization. Adipic acid crystallized in the presence of a variety of stereo-chemically related impurities typifies such behavior with disorder manifest in variations of dissolution rates and enthalpies of solution and fusion. In this case the most extreme habit, produced by the presence of added monoalkanoic acids, is a rounded dumbbell that was suggested previously to be a twinned crystal. In this contribution such crystals are fully characterized both through their external morphology and by means of single crystal X-ray diffraction. These techniques show that these particles are not twinned but rather are disordered single crystals comprising a small number of slightly misaligned domains. The interaction between additive and substrate is modeled and new additives selected that induce the formation of true mechanical twins in adipic acid. (C) 2000 Wiley- Liss, Inc.