A Flory-Huggins-type lattice model of actin polymerization under equilibrium conditions is employed to analyze new spectroscopic measurements for the extent of actin polymerization Phi as a function of temperature T, salt concentration [KCl], and the initial concentration of actin monomers [G(0)]. The theory subsumes existing mechanisms for actin monomer initiation, dimerization, and chain propagation. The extent of polymerization Phi increases with T to an unanticipated maximum, and the calculations explain this unusual effect as arising from a competition between monomer activation, which diminishes upon heating, and propagating chain growth, which increases upon heating. The actin polymerization is described as a rounded phase transition, and the associated polymerization temperature T-p depends strongly, but nearly linearly on [G(0)] and [KCl] over the concentration regimes investigated. Our findings support the suggestion that physicochemical changes can complement regulatory proteins in controlling actin polymerization in living systems.