Changes in the lattice parameter of martensite phase in a maraging steel is measured as a function of ageing temperatures. A decrease in the lattice parameter is observed when aged between 400-650 degrees C which is attributed to the depletion of alloying elements in the martensite matrix as a consequence of precipitation and austenite formation. An increase in the lattice from 650 to 710 degrees C was characteristic of enrichment of alloying elements in the martensite matrix due to dissolution of austenite phase. A minimum value of the lattice parameter of martensite is observed at 650 degrees C which coincides with the maximum vol % of retained austenite. Electrical resistivity, hardness measurements, and in situ high temperature dilatometry was also carried out to delineate various precipitation reactions that occur in this steel.