We report on the absolute determination of the transport mean free Path length of multiple scattered photons by multifrequency phasefluorometry. The time delay between the injection of an ultrashort light pulse and the subsequent detection at a certain separation is measured as a phase-shift induced on the Fourier-components of the light pulse. The diffusion approximation enables the calculation of the number of steps and the transport mean free path from this time delay. Advantages of this technique over the time domain technique [Watson et al., Phys. Rev. Lett. 58 (1987) 9451 are discussed.