The dissociation constants of methyl orange were determined spectrophotometrically in the reverse micelles of CTAB, AOT and Triton X 100. No buffers were used and the acidity of methyl orange solubilised in the reverse micelles was changed by adding hydrochloric acid. The effect of water to surfactant molar ratio(W) and surfactant concentration on pKa values were investigated. All pKa values were lower than in aqueous medium even after taking into account the concentration effect. The shift in pKa at constant W and surfactant concentration follows the order CTAB>Triton×100>AOT. The variation of W has no significant effect on pKa in CTAB whereas in AOT and Triton ×100 the pKa increases on increasing W. The increase in pKa with W is more pronounced at low W values (W<4). The weak acidic nature of methyl orange(pKa=3.6) in aqueous medium is no longer observed in reverse micellar medium and the protonated form of methyl orange acts as a strong acid. In case of CTAB reverse micellar medium this can be attributed to a strong interaction between positively charged surface and negatively charged deprotonated form of dye. The shift in the pKa of the protonated methyl orange in the Triton×100 and AOT reverse micellar medium can be attributed to the more basic nature of the solubilized water, the basic nature decreasing with W.