The thermochromic transition in regioregular poly(3-(2-methyl-1-butoxy)-4-methylthiophene) was characterized by UV-vis, FTIR, and Raman spectroscopies. The nonplanar yellow form of this polymer can be planarized at room temperature upon treatment with acetone vapor to yield a red polymer film. Upon heating, combined spectroscopic measurements have revealed important irreversible color changes (from red to yellow) between 75 and 95 degreesC induced by both side-chain and main-chain disordering. These measurements, in particular Raman spectroscopy, clearly indicate a significant planar-to-nonplanar conformational transition of the polythiophene backbone during the thermochromic transition.