A method is presented for calibration or check of suitable equipment or procedures for measuring the relative humidity generated by the commodities of interest in a closed system. The critical parameters are defined and the control limit for each parameter is established to achieve the desired sensitivity and precision of measurement. A multilaboratory, multi-instrument study of the method, applied to a cross section of commodities and reference standards, demonstrated that measurements of water activity by the described method with instruments using immobilized salt solution sensors can be made with an accuracy and precision within ± 0 . 0 1 , provided there are no commodity-instrument interactions. The study showed that commodity-instrument interactions did exist with lupini beans in brine, soy sauce, and cheese spread, but not with fudge sauce. A sampling error is distinctly evident with walnuts and is probably a component of the variance with rice. There were too few laboratories using other types of instruments than those with immobilized salt solution sensors for a valid statistical analysis of each.