This paper describes a model for the description of the mass transfer from the aqueous sample. Based on the model predictions the applicability of SPME-GC/MS for on-line monitoring and control of water quality in the distribution system is evaluated. The main issue is to gather some information on the influence of hydrodynamics, thermodynamics and kinetics in order to extend SPME's applicability to on-pipe sampling. The model is based on physical mass transfer equations describing the transfer of analytes from the bulk solution to the surface of the fiber, the diffusion of the analyte in the polymer coating of the fiber and the equilibrium factor K of the analyte between the fiber coating and the water phase. The final concentration c2f of the analyte on the fiber is described as a function of the Reynolds number Re (hydrodynamic conditions), the diffusion coefficients d1 (analyte in water) and d2 (analyte in fiber coating), K, the extraction time t, the concentration of the analyte in the bulk solution c1 and the fiber geometry. The experiments were carried out under controlled hydrodynamic conditions in a closed-loop pipe rig in order to verify the theoretical predictions and to determine the rate determining step of the extraction process. The calculations suggest that only substances with similar equilibrium factors should be determined at the same time in order to obtain detector signals of similar size.