Micro- and nanoscale crystals of the thermochromic polydiacetylenes (PDAs) ETCD and PUDO exhibit electronic absorption maxima at wavelengths longer than those observed in bulk crystal specimens of the same materials. This observation is in contrast to the reported behavior of PDA-DCH where the absorption maximum shifts to shorter wavelengths as crystal size decreases. Infrared spectra and X-ray powder data indicate that the small crystals have the same composition and a structure similar to the bulk specimens. Differential scanning calorimetry (DSC) reveals a larger endothermic heat of the thermochromic phase transition in the small crystals vs bulk specimens. The crystal morphology and structure of microsized, nanosized, and bulk crystals of PDA-ETCD and PDA-PUDO were studied by transmission electron microscopy (TEM) and electron diffraction (ED). For both PDAs, no drastic changes in lattice parameters were observed as crystal size decreases from bulk to micro- and nanoscale.