Base equilibria of arginine (basic amino acid) in the absence of a micelle and then in a micellar solution of sodium dodecyl sulfate (SDS) have been studied by ultrasonic absorption measurements. Forward (gamma(2)k(f)) and backward (k(b)) rate constants, the apparent base dissociation constant K-b (=k(h)/gamma(2)kf), and the volume change (Delta V) for the reaction were obtained. Micellar effects for arginine were greater than those for a neutral amino acid measured previously. This is discussed in terms of total charge of the amino acids. Similar experiments were carried out for aspartic acid (acidic amino acid)-cationic micelle of dodecyl-ammonium chloride (DAC). Relaxation was considered due to the protolysis of aspartic acid. A slightly lower micellar effect than that for an aromatic carboxylic acid-dodecylammonium chloride (DAC) system was noted. Delta V was smaller than that for an aromatic carboxylic acid with the same pK(a).