The thicknesses and tensions of black films formed by aqueous solutions of a pure, non-ionic surface-active agent, n-decyl methyl sulphoxide (DMS), have been measured at 298 and 308 K as a function of sodium chloride concentration. In the DMS + NaCl + H2O system, second black films are formed at low ionic strengths whereas first black films are formed at higher ionic strengths (> 100 mol m^{â 3}). Estimated values for the composite Hamaker constant for first and second black films have been obtained and compared with theoretical values.