In this paper we will present a theory on the source of prefixation differences between Polish analytic and synthetic anticausatives. Analytic anticausatives are freely prefixed with superlexical, lexical and ‘pure perfectivizer’ prefixes, while synthetic anticausatives show propensity for ‘pure perfectivizers’, if indeed they are prefixed at all. We have looked for a source of this distinction in OCS anticausative morpho-syntax. We claim that OCS analytic anticausatives are formed within the limits of the voice system of OCS as middle voice word-forms. As such, they have the same rich prefix inventory as other verbal stems that have the same roots, with some of the prefixes introducing changes in verbal lexical meaning. On the other hand, synthetic anticausatives are already at this time members of the OCS lexicon, mostly without any related verbal forms, but sharing roots with nouns and adjectives (for which prefixation is not a frequent operation in Slavic languages). The prefixes appearing with synthetic anticausatives have the function of realizing the viewpoint aspect, rather than word-formational functions. We have traced the distinction between the two classes of anticausatives from the OCS times to Present-Day Polish, quoting also some intermediate stages in the history of these verbs.