We deposited amorphous CuP 2+x thin films with a wide range of x (positive and negative) by reactive sputtering in a PH 3 /Ar atmosphere. Either a metallic Cu target or a compound Cu 3 P target can be used as a deposition source. Some of the amorphous CuP 2+x films could be converted to polycrystalline CuP 2 by rapid thermal annealing in an inert atmosphere at 1 bar. Three conditions must be satisfied to do so: (i) short annealing times, (ii) moderate annealing temperatures in the 350 • C-450 • C range, and (iii) initial composition sufficiently close to the ideal CuP 2 stoichiometry. Remarkably, amorphous films that were either too P-poor or too P-rich quickly decomposed into Cu 3 P and gaseous phosphorus upon heating. This "compositional lock-in" behavior highlights the importance of pre-existing shortrange order for kinetic stabilization of materials under conditions where decomposition and crystallization are in competition with each other.