A solution model previously implemented in the semiempirical molecular orbital method SINDO1 is applied to the study of trimethylenes and tetramethylenes in different solvents. It is shown that diradical trimethylenes and tetramethylenes are polarized by the solvent. This influence of the solvent can also lead to a zwitterionic state in cases where a diradical is predicted without the solvent. Solution effects are the reason for the discrepancy between the experimentally observed polymerization mechanisms of substituted tetramethylenes and gas phase calculations. For a prediction of the correct state, a geometry optimization in solution is important.