The micelles formed by reduction-sensitive amphiphilic copolymer have emerged as promising drug nanocarriers due to the controlled drug release and effective anticancer activity triggered by the reducing stimulation. However, the effect of pH on the stability and guest exchange of the micelles formed by reduction-sensitive copolymer have not been systemically investigated. Herein, the micelles formed by a reduction-sensitive copolymer poly(epsilon-caprolactone)-b-poly[oligo(ethylene glycol) methyl ether methacrylate] (PCL-SS-POEGA) with a single disulfide group at different pH values loaded with dyes 3,3 '-dioctadecyloxacarbocyanine perchlorate/1,1 '-dioctadecyl-3,3,3 ',3 '-tetramethylindocarbocyanine perchlorate (DiO/DiI) were prepared through the precipitation-dialysis method. In addition, mixed micelles formed by different contents of reduction-sensitive and reduction-insensitive copolymers encapsulated with DiO/DiI at pH 7.5 were also prepared by the similar approach. The effects of pH and the content of reduction-sensitive copolymer on guest exchange of these micelles were studied by the fluorescence resonance energy transfer method. Results show that the pH value in the environment has great influence on the guest exchange rate of reduction-sensitive micelles in the presence of 10 mM dithiothreitol (DTT) and slight effect on that in the absence of DTT. Under a reducing environment, the guest exchange rate of the micelles containing various contents of disulfide-linked copolymer increases with the increasing content of PCL-SS-POEGA. (c) 2018 Wiley Periodicals, Inc.