Measurements of light elements in the Solar system can provide important constraints upon their abundances in the protosolar nebula. In the case of helium, different results have been obtained in the four giant planets. These divergences can be accounted for if an evolutionary mechanism, involving helium differentiation in Jupiter and Saturn, is taken into account; if this scenario is correct, the He/H ratio in Uranus and Neptune might be representative of its protosolar value. In the case of deuterium, various determinations derived from planetary and cometary atmospheres show the existence of two deuterium reservoirs: the protosolar nebula, and the interstellar ices(H2O,CH4&ellip;)from which planetesimals accreted. Recent results by Galileo and ISO show that Jupiter and Saturn are the only objects where D/H can be expected to be close to protosolar, while in all other outer solar-system objects (including Uranus and Neptune), deuterium exhibits a significant enrichment due to the contribution of ices incorporated in their nuclei. The3Heabundance in Jupiter, recently measured by Galileo, is believed to be representative of its protosolar value. Finally, lithium has been observed for the first time in a comet, at the time of the collision of comet Shoemaker-Levy 9 in Jupiter in July 1994; however, no information can be derived about its abundance, because the excitation mechanism of the emission line is not understood presently. ©1999 American Institute of Physics.