The disintegration of O/sup 16/ and C/sup 12/ by fast neutrons was studied using a grid-type ionization chamber filled with CO/sub 2/-- Ar or CO/sub 2/-- Kr mixtures. Monoenergetic neutrons for studying the reactions from 5.0--8.8 Mev were produced with the D(d, n)He/sup 3/ reaction using a gas target. The alpha particle group from the O/sup 16/(n, alpha)C/sup 13/ reaction leaving C/sup 13/ in its ground state was studied over the entire energy range. The excitation function showed 21 resonances corresponding to excited states in O/sup 17/. The cross section varied from 2-206 mb. The alpha-particle group leaving C/sup 13/ in its 1st excited state was studied from 7.6--8.7 Mev, the cross section increasing from about 3- 12 mb. Unresolved alpha -particle groups leaving C /sup 13/ in its 2nd- and 3rd-excited states were studied from 8.1--8.7 Mev, the cross section decreasing from an estimated 30--8 mb. The alpha-particle group from the C/sup 12/(n, alpha)Be/sup 9/ reaction leaving Be/sup 9/ in its ground state was studied from 7.9-8.7 Mev. The cross section increased from 60--76 mb at 8.0 Mev and then decreased to 32 mb at 8.7 Mev. (auth).