Polydispersity effects in thermal diffusion forced Rayleigh scattering (TDFRS) on dilute polymer solutions are investigated both theoretically and experimentally and are compared to photon correlation spectroscopy (PCS). Contrary to PCS the statistical weights of the individual molar masses in the TDFRS signal can be changed by variation of the excitation pattern. With short excitation pulses the statistical weights are proportional to the concentration and independent of molar mass. The thermal diffusion coefficient D-T, the weight average Soret and diffusion coefficient, [S-T](c) and [D](c), and [D](c/D), from which the weight average hydrodynamic radius [R(h)](c) is derived, are directly obtained from the experiment. From a deconvolution of the multiexponential decay function the molar mass distribution can be determined. Experiments are reported for solutions of polystyrene in toluene and ethyl acetate and for polystyrene microgels in toluene. The thermal diffusion coefficient of polystyrene in ethyl acetate is obtained as D-T = 1.31 x 10(-7) cm(2) (s K)(-1) (T = 24 degrees C).