Understanding pH effect of cellulolytic enzymes are of great technological importance. In this study, we have examined the influence of pH on activity and stability for central cellulases (Cel7A, Cel7B, Cel6A from Trichoderma reesei and Cel7A from Rasamsonia emersonii). We systematically changed pH from 2 to 7, temperature from 20 - 70°C, and used both soluble (pNPL) and insoluble (Avicel) substrates at different concentrations. Collective interpretation of these data provided new insights. An unusual tolerance to acidic conditions was observed for both investigated Cel7As, but only on real insoluble cellulose. In contrast, pH-profiles on pNPL were bell-shaped with strong loss of activity both above and below the optimal pH for all four enzymes. On a practical level, these observations calls for caution of the common practice of using soluble substrates for general characterization of pH effects on cellulase activity. Kinetic modeling of the experimental data suggested that the nucleophile of Cel7A experiences a strong downward shift in pKa upon complexation with an insoluble substrate. This shift was less pronounced for Cel7B, Cel6A and for Cel7A acting on soluble substrate, and we hypothesize that these differences are related to the accessibility of water to the binding region of the Michaelis complex.