The tools for optimal design of photocatalytic air purifiers are still being established. In this article, the optimal design of monolith photocatalytic reactors irradiated by cylindrical UV lamps was investigated. The radiative transfer equation in dimensionless monoliths was solved by the Monte-Carlo method to yield the radiation field in the reactor. The effect of the dimensionless design parameters, monolith aspect ratio, geometric ratios, catalyst reflectivity and number of lamps were investigated and correlated in terms of photon absorption efficiency (phi), uniformity of radiation distribution (eta), and overall monolith photonic efficiency (Phi = eta phi). Optimal design is achieved for aspect ratios of 2.5 and monolith-to-lamp-length ratios in the range from 1 to 1.1. The largest number of lamps compatible with uniform air distribution over the monoliths should always be used. The optimal monolith-to-lamp-distance ratio, was correlated as: [beta(optimum) = 1.698 (n + 1) - 0.279], where n is the number of lamps.