Low molecular weight ethylene/carbon monoxide (E/CO) copolymers were characterized by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Because of the nominal mass overlap between ethylene and carbon monoxide monomers, the normal copolymer oligomer distribution cannot be observed by time-of-flight (TOF) mass analysis, and no composition or microstructure, information can be obtained. Chemical reduction of the carbon monoxide to a hydroxyl permits observation of an ECO copolymer distribution by increasing the mass of each oligomer in proportion to the carbon monoxide content. Unfortunately, the poor ionization efficiency of the fully reduced copolymer precludes quantitative composition and microstructure measurements. MALDI in combination with Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry permits oligomers having the same total number of monomers but different amounts of ethylene and carbon monoxide to be mass resolved. The discrete oligomer distribution obtained by MALDI-FT-ICR allows E/CO composition and microstructure to be determined. A first-order Markovian model was fit to the experimental data. Oligomers were found to have an alternating sequence of single carbon monoxide monomers between longer lengths of ethylene monomers.