Isothermal mutual diffusion coefficients (D) have been measured for binary aqueous solutions of sodium alkylsulfonates at 25 degrees C. The diffusion coefficient values drop as the concentration of micelles in the system increases. As the length of the hydrocarbon chain increases, the observed drop in D becomes increasingly sharp and shifts to lower concentrations. When the micellization process is treated as a chemical equilibrium, diffusion coefficients lead to the calculation of the thermodynamic parameters association number, n, and the equilibrium constant, K. These parameters are briefly discussed in connection with the alkyl chain length of the surfactants.