Laboratory experiments investigated how the presence of biofilm and particle size influenced deposition in both bare and vegetated channels. Both abiotic and biotic sediment beds were considered, with the biotic beds containing Extracellular Polymeric Substances (EPS). As current magnitude and turbulent kinetic energy increased, particles were more easily resuspended, leading to reduced deposition rate. At the same turbulence level, depositing particles smaller than the sediment grains exhibited higher deposition rates than larger particles because the smaller particles were more effectively shielded from the flow by settling deeper into the spaces between sediment grains. Increasing the concentration of EPS decreased the deposition rate for all particle types, but this trend was more pronounced for smaller particles, because the enhancement in particle exposure due to EPS was more significant. A new model was developed to describe microparticle deposition probability as a function of EPS concentration, flow characteristics, and particle properties.