An evaluation was made of the influence of bioparticle size on the degradation kinetics of a partially soluble synthetic wastewater in an anaerobic sequencing batch biofilm reactor, mechanically stirred, containing immobilized biomass. The 4.5-L reactor was operated at 30 degrees C, with biomass immobilized on cubical polyurethane foam matrices (0.5, 1.0, 2.0 and 3.0 cm sides) and mechanically stirred by three flat-blade turbines (6 cm) at an agitation rate of 500 rpm. The COD of the synthetic wastewater was 1000 mg/L (50% suspended). The apparent first-order kinetic constants (k(1)(app)), calculated for filtered and suspended COD, did not show the expected tendency (smaller the particle, higher the coefficients). For suspended COD, the maximum k(1SS)(app) occurred with 1-cm bioparticles. This particle size probably favoured better foam packing, enhancing the hydrodynamic conditions and dissolution rates. In filtered COD, the maximum k(1F)(app) occurred with 2-cm bioparticles, which was attributed to the correlation between the foam packing and the conventional and nonconventional mass transfer phenomena. Even then, a kinetic analysis of individual substrates (carbohydrates, proteins and lipids) revealed the expected tendency. (c) 2005 Elsevier Ltd. All rights reserved.