The objective of this work is to use direct Monte Carlo techniques in simulating drug delivery from compacts of complex composition, taking into consideration the special features of the in vitro dissolution environment. The paper focuses on simulating a binary system, consisting of poorly soluble drug, dispersed in a matrix of highly soluble acid excipient. At dissolution, the acid excipient develops certain mechanisms, based on local pH modifications of the medium, which strongly influence drug release. Our model directly accounts for such effects as local interactions of the dissolving components, development of wall roughness at the solid–liquid interface, moving concentration boundary layer and mass transport by advection. Results agree with experimental data and have demonstrated that when modelling dissolution in vitro, special attention must be paid to including the particular conditions of the dissolution environment.