The concentration dependence of experimental diffusion coefficients of managanese ions in presence of some alkali metal chlorides is examined in the light of Onsager's theory. The diffusion coefficients are measured in 1% agar gel using zone-diffusion technique. The positive and negative deviations observed at higher and lower concentrations, respectively, are explained in terms of relative contributions of various types of effects occuring in the diffusion medium. Further, the effect of supporting electrolytes on the activation energy and obstruction effect for manganese ions has been studied at 0.1M electrolyte concentration. It is observed that the magnitude of both parameters decreases in the reverse order of the charge density of alkali metal ions of the supporting electrolyte. These observations are explained by considering the distortion in the gel-water structure caused by these ions and agar molecules.