ABSTRACT: This paper presents a simple model to determine the benthic fluxes of matter across the sediment-water interface when a benthic chamber is used. The model is based on accepting that the limiting step for the overall rate of transfer while the chamber isin position is the diffusion through the diffusive boundary layer (DBL) overlying the sediment, and it assumes that this rate is necessarily affected by the procedure employed to measure the flux. Alteration of the benthic fluxes due to the action ofmacrofauna is included in the model and, operatively, is expressed in terms of a reduction in the thickness of the DBL between the sediment and the water. If the diffusive fluxes are determined simultaneously with the benthic chamber experiments, a valuecan be estimated for the thickness of the 'stagnant film'. Owing to the assumptions made and to the micro-heterogeneities of the sediment, this thickness can be considered to have both an operative and a statistical meaning, with respect to the entiresurface area covered by the chamber. The values calculated in the Bay of Cádiz (SW Spain) vary between 0.36 and 0.71 mm, which are in agreement with measurements of this film made by different techniques in other littoral ecosystems. However, the valuesof the in situ fluxes estimated using this model and by other established procedures described in the literature can differ by more than 50%, for the same experiment, in strongly irrigated zones.