Three pairs of end-labeled poly(acrylic acid-b-methyl methacrylate) block copolymers were synthesized to study micelle formation in water by nonradiative energy transfer (NRET). One member of each pair contained naphthalene as the donor chromophore at the poly(methyl methacrylate) (PMMA) chain end, and the other, pyrene as the acceptor. The highest molecular weight sample (70 MMA units) formed micelles at the lowest concentrations we could examine, 0.1 mg/L, and did not undergo chain exchange on the time scale of weeks. This polymer forms micelles when the solid sample is treated with 1 equiv of NaOH in water, but the mechanism of micelle formation appears to involve fragmentation of the polymer powder instead of an association of single block copolymer chains. The samples with 40 or 20 MMA units are largely dissociated in pure water, and micelle formation is only slightly increased at higher polymer concentration. No cmc could be observed. However, for these samples addition of small amounts of salt (NaCl) strongly increases the polymer aggregation. The presence of salt also slows down the exchange rate of polymers among the micelles.