In acetonitrile electropolymerization of pyrrole is considerably facilitated by the addition of a small amount of water. Several mechanisms have been suggested in the past to interpret this dramatic influence of water. However, none of them has explained all the experimental results obtained up to now. On the basis of the mechanism involving the acid-catalyzed formation of a pyrrole trimer, 2,2'-(2,5-pyrrolidinediyl)dipyrrole and its subsequent electropolymerization leading to electrode passivation, it is proposed that water, a fairly strong base in acetonitrile, prevents the acid-catalyzed formation of this trimer by capturing reaction-released protons. This interpretation proved to be consistent with all results. Other substances, such as methanol, ethanol, and tetrahydrofuran, which are more basic than pyrrole, but not basic enough to deprotonate, intermediate species, also have a favorable effect on pyrrole electropolymerization in acetonitrile. On the basis of the data so far obtained, other previously proposed mechanisms can be ruled out.