The barrier to internal rotation in 2-acetylthiophene was calculated by molecular mechanics, semiempirical, and ab initio methods. Comparisons were also made with acetophenone and 2-acetylfuran, as well as with the corresponding aldehydes. Although the calculated barriers were considerably higher for ab initio calculations at the 6-31+G * level, but lower for AM1 than the experimental values, the results were acceptable after suitable scaling. The PM3 and SAM1 methods, however, proved inappropriate for calculations of this kind. MM3 with the Parameter Estimator generally appears to give the most reasonable results.