Solvent-mediated solid-phase transformations may occur during dissolution tests which complicates the evaluation of dissolution rates in cases of metastable drugs. The purpose of this study was to determine the effects of solvent-mediated transformations of theophylline anhydrate (TP (A)) on the intrinsic dissolution rate in simulated gastric fluid at pH 1.2. A combined method set-up for simultaneous measurement of the dissolved quantity of drug and the solid form composition was constructed from in situ Raman spectroscopy and UV-vis-spectrophotometry. Transformation kinetics in the traditional USP rotating disc (RD) dissolution apparatus was compared with the recently introduced channel flow cell (CFC). Solid-phase data, supported by scanning electron micrographs taken off-line, explained the changes in the intrinsic dissolution rates due to hydrate formation. Kinetic modelling showed that first order kinetics fitted the data in CFC, but the conversion in RD was strongly S-shaped. These differences were related to dissimilar hydrodynamic conditions and diffusion characteristics in the two dissolution testing devices. In situ solid-phase measurement during dissolution testing can largely improve the understanding of the dissolution results of metastable drugs. This information is valuable in drug candidate selection as well as in explaining and controlling the behaviour of drug substances in the final drug products.