The diffusion problem of light gases in the atmsophere of Titan is reassessed after Voyager encounter. The pre--Voyager theoretical, work of Hunten (1973a) is completed by the use of parameters derived from Voyager encounter; the radial distances of turbopause level (3500 km) and critical level (4100 km), the exospheric temperature (186 K) and the volume mixing ratio of 2¿10-3 for H2. An analytical expression of the solution of the diffusion equation is given for a spherical geometry and an isothermal atmosphere. It is used to predict an altitude profile of H2 in the upper atmosphere of Titan. The modification of this profile by a possible return flow of H2 from a torus to the atmosphere of Titan is addressed, and is found negligible since there is observational evidence from Voyager that the H2 density is less than 100 cm-3 in the torus, at large distance from Titan. The H2 concentration at the exobase level is 3.1¿105 cm-3 and the thermal escape of H2 is nearly equal to the diffusion limiting flux of 3.4¿109 molecules cm-2 s-1 at the turbopause level. Photodissociation of CH4 has to provide for an equal constant supply of H2.