A reduced kinetic model for the initial steps of the photocatalytic process is presented with the aim to analytically solve the resulting kinetic system. Several possible kinetic models have been explored. Attention was paid to obtain equations with physical meaning and reduced complexity. An analytical equation is obtained for the rate and the quantum yield, which retains the principal features of the photocatalytic process, namely the light induced charge separation and recombination the oxidative and reductive electron transfers, the formation of a stable oxidized intermediate, and, unlike the previous kinetic models, also the back reaction of the oxidized substrate. Compared to the two-parameter Langmuir-Hinshelwood equation, all the previous features and the dependence on the light intensity are described with only three parameters, which collect all the kinetic constants, and account for experimental concentrations of substrate and electron scavenger, light intensity and catalytic system characteristics. The kinetic behavior of photocatalytic systems under all the possible values of experimental parameters can be graphically presented. The analysis of the obtained rate equation shows that the best utilization of photons is attainable at low light intensities, suggesting that preconcentration of solar light is unnecessary.