Physical conditions of high-luminosity star-forming regions observed by ISO-LWS

We present here the results obtained with the Infrared Space Observatory's Long Wavelength Spectrometer on a sample of high-luminosity star forming regions, characterized by the presence of strong far ultraviolet stellar fields causing atomic ionization. We show how photoionization models can explain the observed fine structure line luminosities and where other components, such as photodissociation emission and shock excitation by outflows, have to be included to account for the observed spectra.