For single components fluids, vanishing isothermal compressibility implies that the mass density is constant, but the same conclusion is unknown for multicomponent fluids. Here the volume remains affected by changes of the composition. In the present paper we discuss an apparently natural way to conceptualise, based on derivatives of the Gibbs function g, this ‘volume change due to composition’ as a compression. In this way the phenomenon becomes quantitatively comparable to the usual coefficients of compressibility. This is a first step to investigate the range of validity of the constant density approximation of multicomponent fluids. As an illustration, three different aqueous solutions are discussed.