Reversible pH-induced changes in self-associative behavior in water of random copolymers of 50 mol % sodium 2-(acrylamido)-2-methylpropanesulfonate (NaAMPS) and 50 mol % 6-acrylamidohexanoic acid (AmH), 8-acrylamidooctanoic acid (AmO), or 11-acrylamidoundecanoic acid (AmU) were investigated by various fluorescence techniques with use of fluorescence-labeled polymers. Fluorescence intensities, lifetimes, depolarization, and quenching for the polymers labeled with naphthalene (4 mol %) indicated that all the polymers adopt an open-chain conformation at basic pHs whereas NaAMPS/ AmO and NaAMPS/AmU polymers collapsed into a compact conformation upon a decrease in pH to an acidic region. No such pH-induced conformational change was found for NaAMPS/AmH polymers. Nonradiative energy transfer (NRET) for NaAMPS/AmO and NaAMPS/AmU polymers doubly labeled with naphthalene (4 mol %) and pyrene (1 mol %) indicated a strong tendency for intrapolymer hydrophobic self-association, the polymers with pendent octanoic acid and undecanoic acid groups forming unimolecular micelles (unimer micelles) at acidic pHs. The mean aggregation numbers (N-agg) of these alkyl carboxylic acid residues in the unimer micelles were determined to be 86 and 74, respectively. These N-agg values were fairly close to the numbers of the AmO and AmU units per polymer chain (85 and 69, respectively), indicative of the formation of unimer micelles. The unimer micelles were disrupted into an open-chain conformation when pH was increased to a basic pH, thus exhibiting reversible pH-induced formation and disruption behavior.