We have investigated the enhancement effect on the transport phenomena in solid solutions relative to the pure end members. Only single crystals and measurements carried out in the middle of the intrinsic range have been considered. The enhancement effect for both the self and heterodiffusion coefficients reaches a factor of about 2 around the middle composition. The diffusion data are analysed in terms of vacancies. The correlation effects have been accounted for by making use of Manning's equations for vacancy diffusion in random alloys via unbound free vacancies. We conclude that the major part of the enhancement originates from an increase of the vacancy concentration relative to the pure end members.