To capitalize on the increased processing temperature capability and associated reduction in processing times and costs of high-temperature vacuum carburizing, existing commercial carburizing alloys must be modified for high temperature grain stability while maintaining or improving existing fatigue performance standards. This technology is of particular importance to the manufacture and performance of gears and rotating shafts. It is well established that grain stability may be achieved by small additions of Nb to carburizing steels due to the grain boundary effects of niobium (Nb)-rich carbonitride precipitation. By obtaining a fundamental understanding of the thermodynamic and kinetic behavior of molybdenum (Mo) in high-temperature vacuum carburizing alloys with respect to microalloy precipitate formation, dissolution and coarsening, it becomes increasingly feasible to tailor alloy content and optimize process parameters to retard the onset of abnormal grain growth at increasingly high processing temperatures by manipulation of precipitate distribution and composition. Current research is focused the effect of Mo on the evolution of precipitate distributions and precipitate composition and morphology upon reheat, at a fixed heating rate, to vacuum carburizing temperatures. Precipitate evolution, in turn, will dictate the high temperature grain stability and fatigue performance of experimental alloys. Both quantitative optical microscopy and transmission electron microscopy are used as primary characterization tools. Transmission electron microscopy confirms the presence of greater carbonitride size distribution non-uniformities in Mo-free alloys relative to Mo-containing alloys in both as-received and pseudo-carburized samples. Quantitative microstructural analyses show that the tested experimental alloys containing a nominal Nb addition of 0.10 wt pct greatly retard the onset of abnormal grain growth at 1100 °C. The grain refining effect of a Mo addition of 0.3 wt pct is evident, though the exact mechanism has not been fully established. This project has led to the conclusion that, in addition to possible effects of Mo on Nb diffusivity and Nb-rich particle/matrix interfacial energy, the ability of Mo to refine hot-rolled microstructures and lessen precipitate size distribution non-uniformities may also lessen the observed coarsening rate of precipitates upon reheat due to the creation of favorable precipitate distributions. This effect of Mo then, in turn, leads to improved grain stability and fatigue performance.