Electrospray ionization mass spectrometry provides a highly detailed picture of the various species present in the high energy material poly(3-nitratomethyl-3-methyloxetane), also known as polynimmo. Polynimmo contains up to 18 cyclic oligomers in addition to the dominant tetramer, the highest species detected containing 22 repeat units. Linear oligomers from the tetramer up to species of mass 3200 Da are detected, affording the characterization of several new combinations of end groups X, Y in X-[nimmo](n)-Y. Various cations can be used to generate the cationized species, and highly specific cation-cyclic oligomer interactions are apparent in the adducts with Na+, K+, NH4+ and H+ : some of these can be rationalized through molecular modelling calculations. The cone voltage applied In the e.s.i. experiment is highly significant in influencing the relative abundances of high- and low-molecular mass ions and in the relative amounts of H+ and NH4+ adducts in cationization experiments with NH4Cl. Despite giving highly accurate relative molar masses of individual species within polynimmo, the e.s.i. technique fails to give accurate molecular mass distributions, especially as no doubly- or triply-charged ions are found.