Apparatus for testing the solubility of a medical dosage form includes a chamber (12) for holding a solvent medium (18), in the preferred embodiment a bicarbonate based buffer system. The apparatus also includes a pH probe (66) which is connectable to a supply of carbon dioxide (32, 34), as well as to a supply of helium (40), the supplies being controlled by a control unit (50). The control unit (50) monitors changes in pH of the solvent medium (18) and, as appropriate, feeds pH increasing and/or pH reducing gas from the supplies (32, 34, 40) into the chamber (12). The control unit (50) is able to maintain a uniform pH during testing or to provide a dynamically adjustable pH during testing, for example to three or more different pH levels in order to test the performance of a drug carrier at different levels of acidity or alkalinity for example, mimicking the conditions of the gastrointestinal tract. The apparatus (20) and associated method are particularly suitable for testing and developing dosage forms for oral delivery of drugs.