The present study deals with the participation of fluid phases in the mechanochemical synthesis of phthalylsulphathiazole. Model experiments with optical microscope using crystalline samples revealed the possibility of the interaction between sulphathiazole and phthalic anhydride without their direct contact with each other at a temperature of about 100degreesC. The shift of the reaction temperature into lower-temperature region under mechanical activation of the reagents is demonstrated by means of scanning calorimetry. Assumption concerning the transport of phthalic anhydride through the gas phase as the most probable mechanism of synthesis in the mechanochemical reactor is put forward. (C) 2004 Kluwer Academic Publishers.