We have computed a series of realistic and self-consistent models of the emitted spectra of HII galaxies. Our models combine different codes of chemical evolution, evolutionary population synthesis and photoionization. The emitted spectrum of HII galaxies is reproduced by means of the photoionization code CLOUDY, using as ionizing spectrum the spectral energy distribution of the modelled HII galaxy, which in turn is calculated according to a star formation history and a metallicity evolution given by a chemical evolution model that follows the abundances of 15 different elements. The contribution of emission lines to the broad-band colours is explicitly taken into account. The results of our code are compared with photometric and spectroscopic data of HII galaxies. Our technique reproduces observed diagnostic diagrams, abundances, equivalent width–color and equivalent width–metallicity relations for local HII galaxies.