Thermochromic effects in Langmuir-Blodgett (LB) films of poly(3-alkylthiophenes) mixed with arachidic acid have been studied above room temperature. The temperature-dependent optical absorption, conductivity and time constants associated with them were investigated. The absorption maximum of undoped films shifted to larger energies with increasing temperature. In complementary measurements of spin-coated thin films the shift was observed to occur at a higher temperature than in the LB films. The changes in the optical absorption and the conductivity were only partly reversible and, in addition, time-dependent. The thermal dedoping of doped LB films was also studied.