A process for treating a fluid which comprises the step of exposing the fluid to ultraviolet radiation and high-energy ionizing radiation. It has been discovered that combining ultraviolet radiation with high-energy ionizing radiation in the treatment of a fluid results in synergistic performances of the treatment process--e.g., improvement in the inactivation or killing of microorganisms in the fluid, in destruction of organic contaminants and the like. More specifically, fluid treatment performance is improved to a level typically not possible when using ultraviolet radiation and high-energy ionizing radiation separately.