Thermochromism and optical absorption in mono‐ and multilayers of Langmuir–Blodgett films of poly(3‐alkylthiophenes), poly(3‐octyl‐2,2’‐bithiophene), and poly(3’‐octyl‐2,2’;5’,2‘‐terthiophene) were studied. In sparsely alkylated polythiophenes the magnitude of the thermochromic shift was smaller than in poly(3‐alkylthiophenes) and roughly proportional to the sidechain concentration. Results of Valence Effective Hamiltonian calculations were compared with the experimental results of thermochromism. A vibronic structure was found in the absorption spectra of Langmuir–Blodgett films at room temperature. The vibronic splitting in poly(3‐hexylthiophene) was approximately 0.18 eV as previously has been observed in poly(3‐alkylthiophenes) but in poly(3’‐octyl‐2,2’;5’,2‘‐terthiophene) it was 0.20–0.25 eV. The vibronic peaks stay approximately at constant energies and vanish at elevated temperatures.