The coarsening kinetics of beta prime in Fe-Ni-Al-Mo alloys was investigated with special attention being given to the effects of the lattice parameter mismatch delta and the volume fraction f//v of precipitates. Variation in delta was achieved by varying the content of Mo. In alloys with low f//v, the alloy with smallest delta (0. 06%) shows the slowest coarsening rate. In alloys with high f//v the effect is more subtle. The size distributions of particles in alloys with high f//v show good agreement with the theoretical models proposed by Brailsford et al. and Voorhees et al. ; however, the ratios of kinetic constants for high and low f//v are off by as much as 50%.