This work deals with caking behaviour of lactose monohydrate powders containing small amounts of amorphous lactose. The objective was to elucidate the relation between the caking ability of powder mixtures and the crystallisation behaviour of amorphous lactose. Binary mixtures consisting of amorphous lactose and alpha-lactose monohydrate were prepared and tested using an accelerated caking test. Amorphous lactose samples were prepared using the freeze-drying process. Two caking devices were developed to characterize the caking ability of powders. The effect of amorphous content, relative humidity, temperature and pressure on the caking behaviour of samples was investigated. The results showed that the presence of even small amounts of amorphous lactose (as low as 0.125%) could cause caking. The more influencing parameters were found to be the relative humidity and the temperature whereas the pressure had no significant effect. The caking behaviour was shown to be closely linked with crystallisation extent and kinetics. A dimensionless time based on Avrami model for crystallisation rate was defined allowing unifying the experimental data. (C) 2019 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.