In this work we have analysed and modelled one-minute probability distribution function of solar direct and diffuse irradiance conditioned to the optical air mass. For this purpose, we have used one-minute data acquired in a radiometric station located in southeastern Spain (37.13 degrees N, 3.63 degrees W, 687 m a.m.s.l.). The study has been made over the dimensionless indices k(b) and k(d). We have found marked bimodality in the k(b) distribution function and asymmetry associated with the unimodal function of one-minute k(d) values. In spite of these differences, we have modelled both distribution functions using a functional form based on Botlzmann＇s statistics. These functions have been used in a previous study devoted to modelling the clearness index for global radiation, k(1). In order to model the asymmetry that is evident in the k(d) and k(b) distribution functions, the functional forms have been modified by the inclusion of an additional parameter. The adjustable parameters included in the model equations present a dependence on the optical air mass.