Optimization of the austenite stability and the transformation volume change, both of which depend on the alloy compositions, is analyzed in the context of dispersed-phase transformation toughening associated with the precipitated austenite in high strenght CoNi steels, such as AF 1410. A computer-aided thermodynamics-based procedure has been devised to describe the effect of composition on austenite stability while the effect of composition on the transformation volume change is treated using Vegard's law and the two-γ-states formalism to account for the magnetovolume effects in austenite. A new alloy matrix chemistry Fe-14wt.%Co-15wt.%Ni-2wt.%Cr is proposed, which has a substantially better combination of austenite stability and transformation volume change over that in AF 1410 steel.