The effect of monovalent cation addition on the gamma-Al2O3-to-alpha-Al2O3 phase transition was investigated by differential thermal analysis, powder X-ray diffractometry, and specific-surface-area measurements. The cations Li+, Na+, Ag+, K+, Rb+, and Cs+ were added by an impregnation method, using the appropriate nitrate solution. beta-Al2O3 was the crystalline aluminate phase that formed by reaction between those additives and Al2O3 in the vicinity of the gamma-to-alpha-Al2O3 transition temperature, with the exception of Li+, The transition temperature increased as the ionic radii of the additive increased, The change in specific surface area of these samples after heat treatment showed a trend similar to that of the phase-transition temperature, Thus, Cs+ was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of gamma-Al2O3. Because the order of the phase-transition temperature coincided with that of the formation temperature of beta-Al2O3 in these samples, suppression of ionic diffusion in gamma-Al2O3 by the amorphous phase containing the added cations must have played an important role in retarding the transition to alpha-Al2O3. Larger cations suppressed the diffusion reaction more effectively.