A design of a microelectromechanical systems (MEMS)-based microscale chemical-oxygen iodine laser (μCOIL) system is presented. A mathematical model of the μCOIL system, based upon existing macroscale models of the COIL system and related microscale technologies, is formulated and used to predict system performance. The new μCOIL concept is comprised of an array of cocurrent gas- and liquid-flow singlet-oxygen generators, which supply a supersonic slit nozzle for Iodine dissociation and excitation, a segment of a macroscale optical cavity, and a μ-scale pressure-recovery system to maintain low-pressure operation. Reactor, nozzle, optical cavity, and pressure recovery system models are developed individually and integrated into a model of the overall system. The resulting model of the μCOIL system is employed to determine the optimal reagent throughputs, reactor dimensions, and operating pressures to maximize output energy density, defined as the output power divided by the total system and reagent weight for a 100 s operating time. Detailed simulation results corresponding to an optimal energy density of 24.6 kJ/kg are presented, in conjunction with energy density values obtained over the entire parameter space studied. Consideration is given to the design of a realizable, integrated μCOIL system, for minimum (7.1 kW) and high-power (∼100 kW) systems. Results of the study suggest that the μCOIL system could be a superior alternative to existing COIL devices, via reduced system volume and weight and improved reliability and safety.