The distribution of ammonia with height in the atmospheres of Jupiter and Saturn has been determined by inversion of microwave thermal spectra and from spectroscopic measurements of the Jovian flux in the v 2 vibration-rotation band of NH 3. The uncertainty in both the thermal profiles and brightness temperatures used in the inversion leads to upper and lower estimates of the retrieved NH 3 distributions. In the case of Jupiter, undersaturation of ammonia is very likely in the stratosphere and in the upper troposphere. In the lower troposphere, we find two characteristic regions with the following parameters: 1. (I) 150–175 ⩽ T ⩽ 230–250° K, 0.7 × 10 −4 ⩽ NH 3 H 2 2.0 × 10 −4 , 2. (II) T > 330–340° K, 1.2 × 10 −4 ⩽ NH 3 H 2 ⩽ 1.5 × 10 −3 . These results give further evidence for an altitude-dependent distribution of NH 3 in the deep atmosphere and for the existence of a cloud of nitrogen compounds at the bottom of region I. In the case of Saturn, a large uncertainty still exists, since the possible values of the tropospheric ammonia mixing ratio lie between 1.3 and 7.8 × 10 −4.