The thermochromic and solvatochromic properties of a series of phenylene-based conjugated polymers have been investigated experimentally and theoretically. Poly(1,4-(2,5-dioctyloxyphenylene)2,5-thiophene), poly(1,4-(2, 5-dioctylphenylene)-2,5-furan), and poly(1,4-(2,5-didodecyloxyphenylene)-2,5-(3,4-diethylenedioxythiophene)) have revealed interesting chromic effects which, on the basis of calculations on dimer model compounds, seem to be related to a rather flexible backbone with an energy barrier against planarity <1.3 kcal/mol. However, in the absence of sterically demanding side chains (e.g., poly(1,4-(2,5-dioctyloxyphenylene)-2,5-furan the conjugated polymer can maintain a highly conjugated (coplanar or nearly coplanar) conformation, even at high temperatures or in dilute solutions. Finally, as observed with poly(1,4-(2,5-dioctylphenylene)-2,5-thiophene) if the steric interactions are too strong, no coplanar conformation can be adopted upon cooling or aggregation.