The thermochromic behavior of diacetylene-containing hydrogen-bonding thermotropic Liquid crystalline monomers and a semirigid main chain thermotropic poly(ester-diacetylene) were examined. These materials were successfully designed to undergo dramatic, first-order thermochromic transitions at the onset of their characteristic mesogenic transitions, as exhibited by changes in the shapes and locations of their polydiacetylene absorption bands and the corresponding visually observed color changes. Two different types of chromic response have been observed : one case, the diacid monomer 5,7-DODBA, in which the polydiacetylene chain responds to the liquid crystalline transitions of its host monomeric lattice, and two cases, the 10,12-DHDP monomer and 10,12-DHDP/TC polyester, in which the side chains covalently bonded to the polydiacetylene chain undergo mesogenic transitions that drive the thermochromic transition. It was found that the thermochromic transition is not reversible beyond the melting points of the partially polymerized monomers, whereas the poly(ester-diacetylene) exhibited partial reversibility above the polyester melting point. In all cases, a new absorption curve at higher energies represented the liquid crystalline phase above the mesogenic melt transition.