Microwave spectra of N,N'-dimethylaniline and 4,4'-dimethylarninobenzonitrile have been recorded in a pulsed supersonic jet using chirped pulse techniques. Experimental substitution structures have been determined for both molecules by detection of the spectra of all C-13 and N-15 isotopomers in natural abundance using a broadband spectrometer. Additionally, a narrowband spectrometer has been used to reveal the N-14 quadrupole splittings at high resolution, from which the orbital occupancy numbers of the amino- and cyano-nitrogen atoms have been determined. An apparent direct relationship between these values and the barriers to inversion of the amino groups is discussed.