·tencl J.: Water Activity of Skimmed Milk Powder in the Temperature Range of 20 – 45 °C. Acta Vet. Brno 1999, 68: 209–215. Water activity data for adsorption and desorption of moisture from skimmed milk powder were investigated at temperatures in the range of 20 – 45 o C and moisture content of the material tested from 3.2 to 20 % (wet basis). The experimental procedure used was a gravimetric dynamic method with continuous registration of sample weight changes. Four mathematical models of sorption isotherms (Chung-Pfost, Halsey, Henderson, and Oswin) were evaluated to determine the best fit for the experimental data. The modified Oswin equation was a good model for moisture adsorption and desorption of skimmed milk powder. Water sorption capacity decreased as temperature increased. The critical value of equilibrium moisture content of milk powder tested, corresponding to the water activity equal 0.6, was 11 % (wet basis) at the temperature of 20 oC. Repeated rehydration of the material brought an increase in the original equilibrium moisture content 3.2 % (wet basis) to 6.3 – 8.1 % (wet basis) in relation to the temperature. It was also demonstrated that an increase in equilibrium moisture content was very small (about 4 % wet basis) in the range of water activity 0.1 to 0.9. Higher levels of water activity than 0.9 resulted in a marked increase of equilibrium moisture content and susceptibility to spoilage by microorganisms. The hysteresis effect between moisture adsorption and desorption was insignificant.