In discussions (1) of the influence of structure upon reactivity in the Michael condensation it has been shown that, except in rare cases, the yields of addition products decrease as either the addenda or the acceptors become more highly substituted. In some cases there are unusual differences in reactivity between a substituted active methylene compound and its next higher homolog. For example, benzalacetophenone gave an 80% yield of addition product with ethyl methylmalonate (2) but none with ethyl ethylmalonate (la). If these differences are a result of spatial C,HeCH=CHCOC6H5