The utility of field desorption mass spectrometry (FD-MS) in evaluating organolanthanide-based polymerizations of ethylene is described. The ethylene polymerization reactivity of (C(5)Me(5))(2)Sm, [(C(5)Me(5))(2)Sm(mu-H)](2), (C(5)Me(5))(2)Sm(THF)(2), and related complexes was studied under hydrogen, and oligomers were analyzed which have molecular weights by FD-MS in the 300-1000 La range. FD-MS was used with this system to study the effects of solvent, temperature, and catalyst precursor on molecular weight, termination steps, and end group identity. The FD-MS data showed that neither beta-hydrogen elimination nor solvent metalation was competitive with hydrogenolysis as a termination step under these conditions. Reactions under D-2 led to oligomers which were identified by FD-MS and NMR spectroscopy to cleanly incorporate only two deuterium atoms per polymer chain (i.e., D-(CH2CH2)(n)-D), and reactions conducted with C2D4 under hydrogen formed H-(CD2CD2)(n)-H. No deuterium incorporation from the solvent, toluene-d(8), was observed. The similarity of the FD-MS data obtained from different precursors suggests that each of the above organosamarium precursors leads into the same catalytic cycle. Styrene is not polymerized by these samarium compounds, but FD-MS data on styrene and ethylene reactions showed that styrene could be incorporated into the polyethylene.