Two types of oxygenated collectors were tested against the same emitter, to evaluate their ability to provide stable, enhanced thermionic performance. These two types were: sublimed Mo oxide layers, and a Nb-O single crystal alloy. Initially, the Mo oxide layers provided excellent thermionic performance, but degradation occurred, latter associated with emitter material transport and condensation on the collectors. By contrast, when used in conjunction with an internal getter, the Nb-O alloy resulted in somewhat lower, but stable performance, after an activation process. This activation process was associated with changes in the surface chemistry.