Heterogeneous Equilibrium Without Gas

We know that for homogeneous mixtures the equilibrium constant is determined using both concentration of reactants and of products. For heterogeneous equilibrium involving a gas and a liquid or a solid, we only use the concentration of the gas because the concentration of liquids and gases are constants, and can thus be omitted from the equation. But what of the heterogeneous equilibrium between a solid and a liquid? Both are incompressible, so what do you do?

Thanks!