This study is a contribution for development of in-line monitoring of solid-state forms during pharmaceuticals manufacturing. It concerns the dehydration behaviour of theophylline monohydrate by stirred bed vacuum contact drying. First it was demonstrated that NIRS coupled with PLS regression enabled to measure in-line and in situ the exact composition of the mixture of different forms of theophylline (and water) during drying. Second, the influence of operating parameters (temperature and water activity) on the kinetics of solid state transformations was investigated. It was shown that the dehydration advanced first by the formation of the metastable anhydrate and after a lag time of the stable one. Once the stable appeared, formation of the metastable form came to an end. The temperature was found out to be the main factor controlling the overall process rate but also the filial contents of the stable and metastable anhydrates for the considered dryer and operating conditions range. (C) 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.