We carry out photoionization experiments on laser-cooled alkali atoms by irradiating rubidium and cesium magneto-optical traps (MOTs) with a photoionizing laser beam. Analysis of the loading behavior of the traps allows us to determine photoionization cross-sections for the first excited state of rubidium and cesium based on a trap load/loss model. In addition, our experimental findings reveal systematic peculiarities, which can be ascribed to the low temperature of the atomic samples under investigation.