The optimization of biofiltration technologies can be addressed improving the knowledge of the process taking place within biofilms, mainly biokinetics and mass transport. Biokinetics are usually defined using different methodologies, such as respirometric and titrimetric tests. Mass transport within biofilms is usually described as diffusion through a homogeneous phase, despite it is accepted that biofilms are very heterogeneous. Thus, a quantitative understanding of how biofilm structure is linked to mass transport is essential to develop reliable models. For this purpose different works have collected the results of various diffusion studies, proposing correlations between biofilm density and mass transport. However the reliability of these correlations, widely used in modeling works, is under suspect because data used in their construction are highly dependent on the experimental conditions where they were obtained. The goal of this paper was to experimentally quantify the effective diffusivity inside biofilms, using a specific microsensor, as function of biomass density, for a specific microbial population and substrate. In addition, biofilm diffusivity was measured at different hydrodynamic conditions. Combining both studies, an equation for the calculation of biofilm diffusivity, considering biomass density and liquid phase velocity, was proposed.