The powder metallurgically processed carbon-free alloy Fe-25Co-15Mo (in m.-%) can be hardened via the precipitation of an intermetallic μ phase (Fe, Co) 7 Mo 6 by solution annealing, quenching and subsequent aging. Solution annealing is carried out in the austenite region which leads to a supersaturation of the matrix with molybdenum and the precipitation of nm sized μ phase particles during subsequent aging. “Primary” μ phase particles that are 1–2 μm in diameter formed during prior hot isostatic pressing are the source of molybdenum for aging. However, during solution annealing, these “primary” μ phase particles also impede grain growth. Therefore, the solution annealing process should be optimized to avoid extensive grain growth while still obtaining sufficient molybdenum contents in the matrix. In order to characterize the solution annealing process as a function of time and temperature, dilatometer experiments were performed to monitor the dissolution kinetics of the μ phase. The remaining μ phase fraction was determined in a complementary way by applying two characterization methods: Quantitative analysis of scanning electron microscope images and X-ray diffraction.