Neutron yields of 239Pu-Be(α,n) sources are increasing due to the buildup of 241 Am, an alpha emitter, from the beta decay of 241 Pu. The yields and heat outputs of four sources, in which PuBe13 was formed during fabrication, have been followed for several years. Rates of increase in neutron yields at t = 0 vary from 0.3 to 1.9%/yr for initial 241 Pu contents from 0.10 to 0.70%. For three of these sources the increase in neutron yield follows the radioactive decay of the constituents. For the fourth, the increase in neutron yield is greater than expected. A possible explanation is offered.