We propose to search for PH3 and to determine the C/H ratio in Neptune's troposphere, by an analysis of its infrared spectrum in the regions 4.0-4.3 microns and 6.9-7.5 microns respectively. Phosphine has never been observed in Neptune's atmosphere, but has been detected in Jupiter and Saturn, and its presence in Neptune in observable amounts is expected from disequilibrium thermochemical models.The detection of PH3 will permit us to test the efficiency of the convection within the planet as well as theories of its formation.A PH3 enhancement, with respect to solar abundance, would support, as in the case of CH4, the nucleation model for Neptune's formation scenario. The C/H ratio is known to be enriched in Neptune's troposphere, with respect to the solar abundance, but with a factor 2 uncertainty. Because of CH4 saturation, one has to probe very deep atmospheric levels, at pressures above 1.5b. The best way to search for PH3 is to record the spectrum of Neptune between 4.0 and 4.3 microns. Scattered solar radiation is observed at these wavelengths. The nu1 and nu3 bands of PH3 are both centered at 4.3 microns. A complete scan including the band center and the far blue wing will allow to have maximum sensitivity for the PH3 detection, and, if present, to retrieve its vertical distribution. The best region to measure the abundance of tropospheric methane is the 6.9-7.5 microns, in the far blue wing of the nu4 CH4 band. Thermal emission is expected to be dominant. Both the PH3 and the C/H observations require the use of ISO. The PH3 bands are centered inside the terrestrial CO2 absorption at 4.3 microns. The 6.9-7.5 microns range is not accessible from the Earth, due to terrestrial water vapor absorption.Both observations require the high resolving power of the SWS. In the case of Uranus, these observations will be performed in the Central Programme (Griffin et al). The present proposal will allow a comparative study of both Uranus and Neptune.