Microstructural evolution was investigated in a two-phase gamma-gamma', P/M IN100 alloy subjected to a three-stage heat-treatment. It was found that the cooling rate from the first and highest temperature sets the fractions and morphologies of the primary and secondary gamma' precipitates. Phase distributions varied with location, especially near primary phase interfaces, at the slower rates. Subsequent heat treatment steps did not change the volume fractions of the primary and secondary phases, however the sizes of secondary and tertiary gamma' increased. These observations are discussed on the basis of competing mechanisms of gamma' particle nucleation and/or growth.