When a liquid is quenched, classical nucleation theory asserts that at least a fraction of the solid must nucleate by way of the athermal mechanism. In this letter, we show that this fundamental issue regarding the relative rates of athermal and thermal nucleation during a quench can be readily addressed by comparing the instantaneous quench rate that is in effect at any point during the quench, −dT/dt, to kn∗+/(∂n∗/∂T) evaluated at the corresponding temperature. This and additional considerations permit the construction of a nucleation-mechanism diagram that maps out the thermal-mechanism-dominated and athermal-mechanism-dominated domains in the quench-rate–temperature parameter space. By projecting the thermal trajectory of a quench on the diagram, the relevance or irrelevance of the athermal mechanism, as well as the extent to which athermal mechanism participates in the transformation, can be assessed. © 1998 American Institute of Physics.