The impact of new ionizing fluxes on ISO observations of HII regions and starbursts

Extensive grids of photoionization models have been calculated for single star HII regions and evolving starbursts. We illustrate the predictions for IR fine structure lines which are used to analyse the stellar content, and derive properties such as the age and IMF. The impact of recent ionizing fluxes on the IR lines are shown. First comparisons of our starburst models with IR-diagnostics and the ISO observations of Genzel et al. (1998) are also presented.