Using the MINDO/3 method it has been shown that the two-steps mechanism, proposed in a previous paper1 for the 1,3-dipolar cycladditions, is compatible with several existing experimental results. Firstly, the effects of the introduction of different substituents on the mechanism of the cycloadditions were studied. Secondly, the slight influence of the polarity of the solvent on the reaction rate has been interpreted by solvation calculations. Finally, the stereospecificity observed has been explained. © 1982.