In an attempt to explain the mechanism of thermochromism in poly(3-alkylthiophene)s we conducted lattice energy and Monte Carlo calculations to investigate the conformational structures and conformational energy profiles of the butyl side chains. The results reveal barriers of 0.3-0.6 eV (agreeing with the range found experimentally) and predict stable lattice structures in which the butyl side chains are in low-lying well-defined gauche and cis conformations as well as the lowest energy trans, If a thiophene ring is displaced from its regular position in which it is coplanar with its neighbors and the lattice is then allowed to relax to its minimum energy structure, the ring returns to coplanarity only if its alkyl side chain is in an all-trans conformation. If on the other hand the alkyl bears a gauche or a cis linkage, the ring takes up one of a number of out-of-plane distortions coupled with torsions of several degrees around the bonds linking it with its neighbors. A series of ab initio quantum chemical calculations was conducted to establish the type of distortions of the polythiophene backbone that would reveal che observed thermochromism. One of these is a torsion around the inter-ring bonds; another is a booklike folding of the rings, both of which (among others) were predicted by the lattice relaxation calculations.