The numerical simulation of multicomponent reactive transport has become a dynamic field of subsurface hydrology. Efficient simulation models are necessary which are capable of representing a multitude of parameters influencing geochemical reactions. The mathematical structure of reactive transport problems in porous media is determined by the decision in the chemical model part whether reactions are kinetically controlled or whether a local equilibrium assumption is made between various species involved in the geochemical reactions. The assumptions made in deterministic models are not hold true for reactions involving chemical species of low concentration. Here the emergent phenomenon evolving out of the random fluctuations of the species is difficult to be captured. The stochastic behaviour of the physicochemical processes can be modelled at mesoscopic level by application of stochastic algorithms. The proposed model was verified by comparing the results of Gillespie's algorithms with a deterministic solution for PCE and BTEX degradation reactions. © 2007 Springer-Verlag.