Thermal neutron capture cross sections of 403 ± 8 b. for Pu238, 18.6 ± 0.8 b. for Pu242, and 73.6 ± 1.8 b. for Am243 have been measured by the activation method assuming 36.4 b. for the thermal neutron cross section of Co59. Samples of plutonium (90% Pu242 – 10% Pu238) were irradiated for 1 to 5 months in the NRX reactor. The growth of Pu239, Am243 (the daughter of the 5 hr. Pu243), and Cm244 (the daughter of the 25 min. Am244) was determined by mass analysis and by alpha spectrum analysis and total alpha counting. Resonance capture integrals (from 0.5 ev. to ∞) of 3260 ± 280 b. for Pu238, 1275 ± 30 b. for Pu242, and 2290 ± 50 b. for Am243 were found using cobalt and U238 as standards. Values of 48.6 b. and 282 b. were taken as the resonance integrals of Co59 and U238 respectively.The fission cross section of Pu242 was zero within experimental error.