Degradation kinetics of different size dextrans in a biofilm reactor were evaluated. Degradation rates of dextran standards, measured as time series of oxygen utilisation rates, decreased with increasing initial molecular weight. Removal of bulk phase total organic carbon with time was highly correlated (R2>0.99) and could be modelled with variable half-order degradation rate expressions. A power correlation between initial molecular weight and the variable half-order degradation rate coefficient was found for polymers in the range 6-500 kDa. Degradation of dextran in the colloid size range (MW>1 Mda) did not follow the same kinetics. Reductions in the observed removal rate with polymer size can be explained by the effect of reduced diffusivities of the substrate, without assuming reaction rate effects.