This work deals with the phase transition of anhydrous lactose in solid-state due to moisturizing and subsequent crystallization and caking. It was found that anhydrous lactose is converted to lactose monohydrate when exposed to ambient moisture. The conversion rate and the kinetics of transformation were investigated at various temperatures and RH (15 to 40 degrees C and 70% to 95% RH). The results showed that the kinetic rate and the conversion increase with RH and T. The limiting step was the crystallization of anhydrous beta-lactose. A simple kinetic law for the global rate of transformation was developed. The rate constants and the activation energy were calculated by comparison between the experimental data and the model. The effect of storage conditions on the caking behavior and kinetics of anhydrous lactose was also investigated. The results highlighted that the caking of powders was directly related to the fractional conversion of anhydrous lactose. (c) 2020 Elsevier B.V. All rights reserved.