It can be shown from the masses involved that the reaction O17 (n, α) C14 should be exothermic by 1.72 Mev., and should therefore occur with thermal neutrons. As an attempt to detect the reaction an investigation was made of the pulse spectrum produced when an ionization chamber containing oxygen was placed in a flux of thermal neutrons. The pulses were amplified and then counted in a series of energy bands by means of a kicksorter. Two oxygen samples were compared, one being enriched (three times normal), and the other impoverished (0.5 times normal), in O17. Differences between runs with these two gases showed a considerable number of pulses between 1 and 2 Mev., with a maximum at 1.4 Mev. Reasons are given for assigning these pulses to the above reaction, including considerations of the possibility that other exothermic neutron reactions occur. By comparing the number of pulses with the number produced by N14 (n, p) C14 when the chamber contained air, a value of (1.9 ± 0.5) × 10−28 cm.2 for the cross-section for the O17 (n, α) C14 reaction in ordinary oxygen was deduced. This gives a cross-section of (0.46 ± 0.11) × 10−24 cm.2 per atom of the pure isotope O17. The effect measured is about 20% of the total thermal neutron capture cross-section that has been reported for ordinary oxygen.