The water activity of four salt slurries (barium chloride, potassium bromide, cobalt chloride and sodium bromide) and four foods (cheese spread, fruit preserves, chocolate frosting and fudge sauce) was determined 10 times at approximately 20°C. 25°C and 30°C using an electric hygrometer. In general, an increase in temperature resulted in a decrease in water activity. The magnitude of the decrease was typically greater between 25°C and 30°C than between 20°C and 25°C. This decrease was also greater when the substrate under test was in the lower aw range. Thus temperature control is very important in inter-laboratory comparisons and when measuring water activity levels near critical values.