Reverse micellisations of nonionic surfactants (Tween 80 and Span 80) in various pure nonaqueous solvents like carbontetrachloride, benzene, toluene, chloroform, cyclohexane and xylene, and in mixed solvent systems like CCl4-DMF and CCl4-DMSO covering the low dielectric range, are studied by UV electronic spectroscopy at 25°C. Micellar aggregation number (n s ) in various solvents is evaluated using the hydrophobic dye, Oil Red. 0. The critical micellar concentration (cmc) andn s ) dependencies on the solvent dielectric, solvent ionization potential, solvent-surfactant interaction potentials, mole fractions of solvents in mixtures and internal pressures are studied. The CCl4-DMSO system shows acmc increase effect, while the CCl4-DMF system shows acmc decrease effect and the excess functions of CCl4-DMSO and CCl4-DMF show increase and decrease effects respectively. Rationalisations of the results and the role of the solvent cage effect have been discussed.