Partially cooperative phase transformations, with characteristically sigmoidal conversion curves, are commonly observed, but rigorous analytical solutions are widely familiar only for fully cooperative and fully noncooperative conversions (exp(−Kt4) and exp(−kt), respectively, in three dimensions). The JMAK formula, exp(−κtn) with noninteger Avrami exponent n, has been used to fit data for partially cooperative conversions, but this approach has only been empirical and so far seems to lack theoretical derivation and support. We show that the Ishibashi–Takagi modification of Avrami theory rigorously accounts for partial cooperativity that arises from the competition between random volume filling by newly formed nuclei of finite volume and cooperative domain growth. The imperfect cooperativity and finite initial domain volume are accounted for by a prenucleation growth time t0, resulting in conversion curves of the …