Phase development was studied in sputter-deposited and annealed ZrO2-TiO2 nanolaminate films on fused silica substrates. The goal was to understand phase evolution as these structures moved toward thermodynamic equilibrium. The results show that diffusive amorphization to form alpha -ZTO was the first reaction of the as-deposited constituents at low temperature (700 degreesC). Growth of alpha -ZTO was self-limited, and a second metastable reaction product with an incommensurate alpha -PbO2-type lattice, alpha -ZTO, formed with time at temperature. Terminal phases formed only when the annealing temperature was raised to 1000 degreesC, thereby lifting kinetic constraints to diffusion. The terminal phases were ZrTiO4 or ZrTi2O6, depending upon film stoichiometry. This study demonstrates that in the absence of a physical driving force to promote layer registration upon low temperature annealing, constituents react to lower the system's free energy via a series of metastable phases that involve limited atomic rearrangement. Equilibrium phases are formed only after the kinetic constraints to diffusion are relaxed.