|We know relatively little about the relationship between verbs with the suffix -nu-, such as крикнуть ‘shout once’, and verbs with the prefix s-, such as сглупить ‘do something stupid’ and сходить ‘walk someplace and back once’. In her Cluster Model of Russian aspect Janda (2007) claims that there is a single group of such Perfectives, namely the Single Act Perfectives. In order to evaluate this claim, we have gathered a database that includes both types of Single Act Perfectives and undertaken a statistical analysis. On the basis of this analysis we show that the distribution of the -nu- and s- morphemes is very much dependent on the morphological class of the verb. In addition, we have traced the historical development of the meaning of semelfactivity for the prefix s- and our study shows that this development comports well with the development of actional meanings for prefixes in Russian in the sixteenth and seventeenth centuries. This article presents new data concerning semelfactive Perfectives in Russian, along with an analysis of this data. The article raises questions about the historical development of aspect in Russian and about allomorphy in general.