Thermochromism in the polydiacetylenes(PDA) from the alkylurethanes of 5,7-dodecadiyn-1,12-diol is associated with a first order phase transition involving the expansion of the crystallographic unit cell, the preservation of the urethane hydrogen bonding, and possibly some relief of mechanical strain upon heating. Insights into thermochromism obtained from recent studies of nonthermochromic forms of PDA-ETCD are presented. Initial observations of the surfaces of single crystals of several PDA in this class by atomic force microscopy are consistent with known crystallographic data.