Lattice parameters, microanalysis, and microstructures of phases present in solution treated and aged samples of Ni-9 at. % Sn and Ni-9 at. % Sn-10 at. % Mo alloys have been investigated. Cellular precipitation was observed to occur in both alloys. During the initial stages of transformation the composition of the α-phase constituting the cells did not change with time. However, on formation of secondary and tertiary cells with interlamellar spacings much higher than those of the primary cells, the tin content of the α-phase of the cell decreased continuously with time and approached the equilibrium composition. The loss of supersaturation in the initial stages increased with decrease in temperature. Almost all of the molybdenum remained in the α-phase which contains 6.7 at. % Sn and 10.3 at. % Mo. Very little molybdenum partitioned to the compound which contained 24.9 at. % Sn and 0.40 at. % Mo. The presence of molybdenum considerably decreased the rate of nucleation and growth, reduced the interlamellar spacing, and increased the loss of supersaturation. However, the increase in hardness was observed to be small which is attributable primarily to solid solution hardening.