The present study considers a scale-up methodology for photocatalytic slurry reactors. The Photo-CREC Water units used include: (a) a 2.65 L internally irradiated annular photoreactor, (b) a 9.8 L externally irradiated scaled-up unit. The LVREA (Local Volumetric Rate of Energy Adsorption Field) calculations for the bench-scale and the scaled-up reactors at different operation conditions are determined by using approaches established by Valades-Pelayo et al. [40] and Valades-Pelayo et al. [41]. In the bench-scale photoreactor, degradations of oxalic acid are carried out at different photocatalyst concentrations and lamp emissions. Residence time distributions are determined for both the Photo-CREC Water II and the Photo-CREC Water III by using glucose as a tracer. An efficiency factor is calculated in both cases including mixing mechanisms and charge! recombination phenomena using a simplified kinetic model. To avoid cross-correlation issues, all relevant parameters are determined by independent experiments. Model validation is also accomplished by comparing model predictions to experimental degradation rates at different photocatalyst concentrations in the larger Photo-CREC Water III. The proposed methodology confirms the applicability of reaction engineering principles for scale-up, from bench to pilot-plant photoreactors. (C) 2015 Elsevier B.V. All rights reserved.