Herein we describe a new experimental protocol and develop a corresponding kinetic model of phase change that together enable the decoupling of simultaneous nucleation and growth processes and quantification of their kinetics. Growth is effectively isolated from nucleation by dividing a phase transition into two isothermal stages: prenucleation, where product crystallites nucleate and grow concurrently, and growth, in which transformation is completed essentially entirely by the expansion of these "seed" grains. Using the model, the temperature dependence of growth may be extracted from converted fraction versus time data for specimens that are appreciably larger than the mean crystal grain size. Similarly, the growth velocity can be ascertained from thin films that exhibit thickness-dependent transition kinetics owing to geometric constraints imposed by the specimen size. The combination of grain growth kinetics and information derived from the transformation of unseeded materials then allows the subsequent calculation of the temperature dependence and absolute rates of nucleation. A preliminary assessment of this method (for the crystallization of amorphous solid water) indicates that the model captures the basic physics of "seeded" transformations and that the technique appears to be a viable method of quantifying simultaneous nucleation and growth in some solids. (C) 2003 American Institute of Physics.