A scale-up model for photoreactors based on a comparative study of the photocatalytic efficiency of suspended and immobilized systems was developed. The model is independent of reactor size and configurations, and it assumes that photocatalytic efficiency is the same when normalized per unit of illuminated catalyst area in both systems. In all cases, phenol/TiO2 (Degussa P25) was selected as the photodegradation system. First, a kinetic model was built in an immobilized system based on the corresponding experimental data, and then predicted rates of phenol degradation in the suspended system were calculated using the above kinetic model combined with a simplified radiation model, which was expressed as an apparent form of the Lambert law. Second, to obtain experimental rates, experiments conducted in the suspended system were carried out under the same conditions used in the immobilized system. Ratios between experimental rates and predicted rates were obtained, revealing the differences in efficiency between the suspended and immobilized systems. The typical value of the ratio was 2.5-9.2, suggesting that the efficiency of the suspended system was 2.5-9.2 times higher than that of the immobilized system. The ratio decreased with increasing concentrations of both phenol and catalyst. When the catalyst concentration and initial concentration of phenol were set, the ratio became constant within the range of the light intensity of 1.71-3.60 mW cm(-2). Finally, for photoreactor scale-up, the proposed model was validated in a larger photoreactor operated in the suspended system, and good agreements were obtained with errors less than 5%. This methodology provides an alternative to the scale-up of photoreactors, which allows for easier engineering applications.