This paper studies the radiant energy distribution inside a multilamp tubular photoreactor used for water and wastewater treatment. The system is a continuous tubular reactor irradiated from outside by four germicidal lamps symmetrically located about the reactor. The equations governing the energy transfer were formulated and solved numerically for two limiting cases: (i) a diactinic medium and (ii) a medium for which the attenation coefficient is assumed independent of the extent of the reaction. To do so, the Extense Source model with voluminal emission was applied. The radiation field inside the reactor was then analyzed as a function of position and of the radiation absorbing characteristics of the medium. Through the analysis of the results obtained in this work, it is possible to draw useful criteria to improve the design and operation of this type of photoreactor.