The mobilities of radical ions of a series of organic compounds in n-alkanes with viscosities within the range of 0.2-4 cP were determined by applying the method of time-resolved electric field effect. The obtained data were used to express the correlation between the mobilities and solvent viscosity in the form of the modified Stokes-Einstein relation. The relation was parametrized in such a way that the specific molecular properties of both solvent and solute appear in the expression only as the ratio of the volumes of their molecules. A significant difference between aromatic and aliphatic compounds was found with respect to the dependence of radical ion mobility on this volume ratio, and two different parametrizations were suggested for mobility estimation in these cases.