Rates of evaporation of Ba and Sr from oxide cathodes in a practical tube structure were measured during life tests up to 20 000 hours. The effects on evaporation rates are shown for (a) impurity in the Ni support, (b) cathode temperature, and (c) space current. We conclude:1. The product evaporating from commercial type cathodes under normal conditions is largely Ba metal; less than 5 percent is Sr; less than 2 percent is BaO; and less than 0.01 percent is SrO.2. Chemical reducing agents in the support metal‡ are important in determining the rate of Ba evaporating during life.3. A factor not closely controlled in the present experiments—anode and grid composition—affects the rate markedly. This is especially true for the evolution of Ba during exhaust.4. Within the precision of these experiments, there is (a) no correlation between the rate of Ba evaporation and thermionic activity of individual cathodes, and (b) no effect of space current on the rate of Ba evaporation.