Phase separation in polymer solutions is highly asymmetric : an inhomogeneous dilute phase of isolated coils is in equilibrium with a homogeneous concentrated solution. Many deviations from the simple Flory-Huggins predictions, found experimentally, can be traced back to this fact. Here, a previously introduced improved description obtained by an explicit coupling of configurational statistics to thermodynamics is extended to polydisperse polymer solutions. It is shown to predict a chain length dependence of the distribution of polymer molecules over the two phases that differs from the original F-H predictions but agrees well with experimental results.