The brightness temperature of Uranus at 20 cm is 260¿16 K, while for Neptune it is 318¿10 K. Since NH3 is the dominant absorber at this wavelength we have modeled the microwave spectra of Neptune based upon an assumed deep gaseous mixing ratio of NH3 and subsequent loss into clouds. The difference between the two brightness temperatures implies that the NH3 mixing ratio below the level of cloud formation on Neptune compared to Uranus is lower by nearly 2 orders of magnitude. An alternative explanation is that the 20 cm radiation from Neptune is a combination of thermal plus synchrotron emission as proposed by de Patter and Goertz [1989>. ¿ American Geophysical Union 1989