A cesium surface ionization source of the type and geometry customarily used in conjunction with sputter-type negative heavy ion sources has been characterized. Measurements have been made of positive-ion production and probabilities of ionization as functions of extraction voltage and cesium oven temperature for ionizer porosities of 0.7 and 0.8. The perveance P of the source, when operated in the space-charge-limited regime with the ρ=0.7ρ0 ionizer, is found from experiment to be 7.61×10-4 μP, while that for the ρ=0.8ρ0 ionizer is 1.92×10-4 μP. These values are lower by factors of 3.88 and 15.53, respectively, than those predicted by numerical solution to Poisson's equation for full area emission from the source. Positive-ion current versus ionizer temperature data are also presented along with mechanical design features of the source.