Base-catalyzed degradation (isomerization) of prostaglandin A2 (PGA2) to prostaglandin B2 (PGB2) was measured at pH 12 and 60°C, and the effect of micellization of PGA2 on the degradation was investigated. The rate of degradation was determined by measuring the rate of appearance of PGB2 with a spectrophotometer. The apparent rate constant, kapp, for degradation of free PGA2 was approximately constant below the critical micelle concentration (cmc), while kapp was decreased by the micellizatton of PGA2. This is considered to be due to the electrostatic repulsion between OH- and negatively charged PGA2 micelles. The value of kapp of PGA2 above the cmc was slightly increased by the addition of sodium chloride as an electrolyte. The apparent rate, μ, for degradation of PGA2 above the cmc could be explained in terms of the sum of two degradation rates, μf and μm, in the bulk and micellar phases, respectively : the increase in μ by the addition of NaCl was due to the decrease in μf and the increase in μm. For PGA2-nonionic surfactant mixed micellar systems, kapp was decreased as the mole fraction of PGA2 increased. These phenomena could be explained by the micellar surface potential in conformity with the Gouy-Chapman theory.