Values for the D/H ratios of Jupiter and Saturn predicted by models of their interiors (Guillot 1999) and inferred from observations of HD in their atmospheres. The D/H ratio of the protosolar cloud inferred from the solar wind by Geiss & Gloeckler (1998) and its error bars are also indicated using a black solid line and black dashed lines, respectively. Left panel: values for the Jovian D/H ratio based on observations with the Mauna Kea Observatory, Galileo/GPMS, ISO/SWS and Cassini/CIRS were taken from Smith et al. (1989), Mahaffy et al. (1998), Lellouch et al. (2001) and Pierel et al. (2017), respectively. Right panel: values for the Saturnian D/H ratios inferred from ISO/LWS, ISO/SWS and Cassini/CIRS data were taken from Griffin et al. (1996), Lellouch et al. (2001) and Pierel et al. (2017), respectively.