An atomistic Monte Carlo (MC) method has been used to predict equilibrium segregation of isovalent cations to (001) surfaces in (Fex.Mn-x)O and (NixCol-x)O. The surface is found to be enriched with solvent in both systems. Long-range electrostatic interactions and atomic motions that occur on small time scales make the MC approach very computationally demanding. The Free Energy Minimization (FEM) method is a more efficient alternative for performing such segregation simulations, but involves several approximations. Comparison of the surface segregation profiles determined using the MC and FEM simulation methods show that the two are essentially indistinguishable. The FEM results can be obtained about 1,000 times faster than the MC predictions. Therefore, the FEM method is a practical and accurate alternative to the more cumbersome MC approach.