Stability and moisture content of ascorbyl monopalmitate (AsA-MP), ascorbic acid (AsA) and their diluted powders were investigated after storage for 2 weeks at 37° in various relative humidities (20, 50, 70, 100%). Diluted powders were prepared by mixing AsA-MP and AsA with phenacetin and lactose, all in the same quantity (100mg). The residual rate of AsA-MP and its diluted powders was determined by selenium dioxide method. The data obtained indicated that the decomposition of the two compounds and their diluted powders was accelerated in parallel with the increase in moisture content. AsA-MP was more stable than AsA in every degree of relative humidity. The moisture content of AsA-MP in various relative humidities was lower than that of AsA.Diluted powders of the two compounds showed corresponding tendency both in stability and in moisture content. Further, the logarithm of residual rate of AsA-MP in various relative humidities changed in direct proportion to the moisture content, but AsA did not.