The dynamics of biofilm formation on polyethylene bioparticles in mesophilic anaerobic conditions, using an inverse fluidized bed as immobilization system, have been studied. The immobilization process was carried out using acetate and glucose as carbon sources. Scanning electron microscopy (SEM) showed cocci cells and Methanosaeta rods embedded into an exopolimeric substances (ESP) matrix. The acetoclastic activity was found to increase as soon as the lag biomass accumulation phase ended. The rate of immobilization was found to conform to a pseudo first order kinetics with good correlation. This kinetic model was used to estimate rate and equilibrium constants for the immobilization process. Rate properties have been explained in terms of attachment and detachment processes.