The electrical conductances, UV-visible absorbances, and osmotic coefficients of aqueous solutions of methyl orange, sodium 4-(4′-dimethylaminophenyldiazo)-phenylsulfonate, have been measured at 25°C. From the behavior of these quantities as a function of concentration it is concluded that the methyl orange anion is almost completely dimerized above 0.2 mM and undergoes further aggregation above 1 mM. Literature values of the osmotic coefficients of aqueous methyl orange at 50 and 60°C are fitted to the monomer-dimer equilibrium yielding values of the dimer formation constant, Kd. Extrapolation to 25°C yields an estimate of Kd=9200 at this temperature. This result is shown to be consistent with the conductance data at 25°C presented here.