The critical micellar concentration of sodium dodecyl sulfate is strongly altered by tris(hydroxy-methyl)methylammonium ions. The effect of buffer solutions containing this weak electrolyte as the counterion source has been studied using various concentrations of the acid-base system as well as modifying the pH. Results show that counterion concentrations ranging from 0 to similar to 340 x 10(-3) M induce an appreciable diminution of the critical micellar concentration from similar to 8 to similar to 0.7 x 10(-3) M. The analysis of data suggests that the critical micellar concentration of sodium dodecyl sulfate depends on the concentration of weak electrolytes in a way very similar to that of strong electrolytes.