Abstract Spectra of Uranus and Neptune have been obtained in the 190–310 GHz frequency range (i.e., 6.3–10.3 cm −1, wavelength range 1.0–1.5 mm), with a Fourier transform spectrometer at the Caltech Submillimeter Observatory. The data, with a spectral resolution of 7.2 GHz, were analyzed in a search for CO and PH 3. Neither the CO (2–1) transition at 230 GHz nor the PH 3(1–0) transition at 267 GHz were detected. The absence of a detectable absorption feature at the position of the CO (2–1) line favors a CO tropospheric mixing ratio lower than 5 × 10 −7in Uranus and 1.0 × 10 −6in Neptune (3σ limits). In the case of Neptune, a tropospheric mixing ratio of 6 × 10 −7seems to be in best agreement with all the existing low resolution data for CO. In the case of Uranus, a PH 3enrichment by more than a factor of 4 can be excluded. There is no evidence for strong PH 3supersaturation in either Uranus or Neptune.