The design of an optimized resonator for a supersonic chemical oxygen iodine laser (COIL) requires detailed information of the saturated gain distribution. By using diaphragms with slit apertures of various sizes at different positions in a stable resonator configuration it was possible to work out detailed information on the coupling characteristics and the influence of iodine repumping and deactivation processes. The measurements were performed at the DLR Lampoldshausen 10 kW supersonic COIL. A high Fresnel number stable resonator is coupled to the active medium and extends about 40 mm downstream from the exit plane of the expansion nozzle array. Apertures with slit widths ranging from 14% to 100% of the full resonator width were introduced into the cavity directly in front of the outcoupling mirror at a fixed z-position of 17 mm downstream of the nozzle exit. The center of another aperture with a fixed slit width of 10 mm could be positioned in flow direction from near the nozzle exit plane to about 29 mm downstream. The optical alignment was controlled with respect to the laser beam size and power density distribution at the outcoupling mirror. As a function of slit aperture the laser output power saturates well before the aperture of the resonator is fully opened. The experimental results are compared with calculations based on the theoretical model of Hager et al. Measured pressure values along the flow direction from the nozzle exit plane to well beyond the laser cavity show a strong dependence on position and width of the diaphragm, resulting from deactivation of unexploited singlet delta oxygen. After taking these pressure effects into account good agreement is found between theory and experiment. The data highlight the significance of deactivation processes for the achievable output power.