The neutron yields of plutonium-beryllium (α,n) sources, made of nominal 239Pu, are increasing due to the buildup of 241Am, an alpha emitter, from the beta-decay of 241Pu which is present in these sources. Measured increases in yields are compared with predicted increases based on the isotopic composition of the plutonium used in these sources. In cases where the isotopic composition was not previously known, the composition was deduced from calorimetric measurements. Sources were found to have initial rates of increase of up to 2% per year. In all cases except one the measured rate of increase was in agreement with the predicted rate.