The effects of two methylated-β-cyclodextrins, heptakis (2, 6-di-O-methyl)-β-cyclodextrin (DM-β-CyD) and heptakis (2, 3, 6-tri-O-methyl)-β-cyclodextrin (TM-β-CyD), on the dehydration rate of prostaglandin E₂ (PGE₂) and the isomerization rate of prostaglandin A₂ (PGA₂) in aqueous alkaline solution were investigated in comparison with natural β-cyclodextrin (β-CyD). In contrast to the acceleration effect of β-CyD, both DM-β-CyD and TM-β-CyD significantly retarded the reaction rates. The stabilizing effect of DM-β-CyD was larger than that of TM-β-CyD. Stability constants and rate constants of the complexes were kinetically determined on the basis of 1 : 1 inclusion complex formation. The data suggest that DM-β-CyD is useful in the stabilization of PGEs and PGAs in aqueous solution.