Recent research has demonstrated that matrix-assisted laser desorption/ionization (MALDI) is feasible for Fourier transform mass spectrometric analysis of biomolecules with masses in excess of 50,000 Da. Here, the effects of electrostatic deceleration times and laser energy upon mass resolution are reported. It is demonstrated that optimum deceleration times for singly-charged MALDI-generated protein ions ranging in mass from 2627 to 29,000 Da are a linear function of m1/2 when a 9.5-V decelerating potential is used. Furthermore, higher resolution is obtained with laser fluences close to the threshold for MALDI. Slow metastable decay of molecular ions in the absence of co-matrix is demonstrated for melittin and bovine insulin. It appears that the resolution enhancing effect of co-matrix may result from slowing molecular ion unimolecular decomposition rates sufficiently to allow infrared emission to compete with metastable decay, thus providing the requisite population of long-lived ions for high mass resolution. A spectrum of bovine insulin molecular ion with mass resolution of 30,000 is presented, together with several spectra of lower mass proteins with mass resolution in excess of 100,000. Detection of a doubly-charged carbonic anhydrase trimer ion with a mass of 87,000 Da is reported.