We investigate the depletion induced phase separation in mixed suspensions of colloidal spheres with diameter sigma and colloidal rods with length L and diameter D. Using an extension of the free volume method developed by one of us [H.N.W. Lekkerkerker and A. Stroobants, Physica A 195, 387 (1993)], we calculate the phase diagrams for L less than or equal to sigma and various L/D ratios. For long thin rods, the volume fractions of rods needed to induce phase separation scale as D-2/L sigma. Good agreement with recent experimental results is obtained.