In order to determine the coexisting phases in a colloid polymer mixture, exhibiting a gas liquid phase transition, a new method is presented making use of volume measurements. By combining the information from the volume measurements along at least three different paths through the phase diagram, and the lever rule, the binodal and the associated tie lines are reconstructed. Four samples with different concentration ratios of hard sphere colloidal silica particles (radius 47 nm) and nonadsorbing polymer (polydimethylsiloxane, radius of gyration 23 nm) were studied. The general shape of the phase diagram is qualitatively in agreement with other experiments and theory.