Atmospheric pressure chemical ionization mass spectrometry (APCI-MS) has been used to characterize the airsensitive paramagnetic organouranium azide and nitride complexes [(C5Me5)(2)UN3(mu-N-3)](3) and [(C5Me5)U(mu-I)(2)](3)N, respectively. The trimetallic complex [(C5Me5)U(mu-l)(2)](3)E had been identified by X-ray crystallography, but the data did not definitively identify E as N3- versus O2- or (OH)(-), a common problem in heavy-element nitride complexes involving metals with variable oxidation states. A comparison of the 250 degrees C APCI-MS spectra of products made from NaN3 and (NaNNN)-N-15 showed mixed [M](+) and [M + H](+) envelopes at expected ion intensities for the N-14 and N-15 isotopomers. A compilation of U-C(C5Me5) and U-I bond distance data for U3+ and U4+ is also reported that shows that the ranges for the two oxidation states have significant overlap.