A new linear regularity recently reported for pure dense fluids, that (Z - 1)nu(2) is linear with respect to rho(2), is found from experiment to be valid for mixtures as well. A simple model that mimics the regularity is used to predict the composition and temperature dependences of the two parameters of the linear isotherms. The results are used to predict the densities of some binary mixtures at different compositions and temperatures; agreement with experiment is better than 1%. Also, the density of a ternary system is calculated and agrees with experiment within 1.5%. The predicted composition dependences of the parameters of the linear isotherms are found to be accurate even for systems in which a stable complex forms, if the complex is considered as a separate species.