Commercially available microcrystalline cellulose (MCC) preparations are widely used as substrates for the characterization of cellulolytic enzymes. The intent of this paper is to demonstrate the importance of considering particle size when using these substrates and to provide a rationale for particle‐size related phenomena. Rates of saccharification of MCC substrates having average particle sizes of 20, 50 and 100 μm were compared under typical Trichoderma reesei cellulase‐based saccharification conditions. The results demonstrate that MCC substrates of different particle size are not equivalent with respect to their kinetics of enzymatic saccharification. The three MCC substrates were shown to be essentially equivalent with respect to physical and chemical parameters commonly correlated with rates of saccharification. Hence, the influence of particle size on rates of MCC saccharification is likely due to the relative importance of mass transfer limitations within the agglomerate particles that make up these substrates.