We wish to determine the emission line ratios of infrared lines
 important in the cooling of metal-rich giant HII regions in the nearby
 spiral galaxies M83 and M51. These line ratios, which only ISO
 has the capability to measure in the near future, will allow us
 to fit detailed theoretical models of the HII regions, and:
 (i) determine, for the first time, accurate chemical abundances
   in the gas at metallicities higher than solar,
 (ii) very greatly improve the calibration of the important optical
   "strong line" abundance indicator for giant HII regions at high
   metallicities, with considerable benefit to the whole field
   of chemical abundances and chemical evolution in galaxies,
 (iii) investigate the extent of depletion of the refractory elements
   Si and Fe onto grains in giant HII regions, and the relative
   importance of infrared emission lines of Si and Fe as coolants
   in metal-rich nebulae.