This work focuses on the influence of the source of organic matter on the process of biomass adhesion on polyurethane foam matrices in fixed-bed anaerobic immobilized-sludge reactors. Five experiments were performed in differential 'gradientless' reactors fed with meat extract (protein), glucose, starch, lipids and complex substrate. The polyurethane foam colonization process was monitored temporally in each experiment to identify the amount of biomass buildup, extracellular polymer production and the morphological characteristics of the cells adhering to the support. Different immobilization patterns were observed for the different substrates used. The morphological variety was found to be dependent on the substrate constituents. Polymer excretion was apparently crucial in the colonization process of the polyurethane matrices and was likely related to cell fixation on the support. The production of extracellular polymeric substances speeded up the initial fixation of microorganisms on the polyurethane surface.