[en] Silicon carbide and nitride have been attracting a special attention as heat-resisting materials in relation with the energy problem. The oxygen in these materials is considered to be related with their thermal-shock-resisting ability. If ordinary methods are employed for the analysis of the oxygen in such materials that contain silicon as a matrix element or as a major constituent, there arise many difficult problems yet to be solved. It is expected, however, that these material can be analyzed rather rapidly, non-destructively and accurately by the 14 MeV neutron activation analysis if a proper consideration is given to the random sum of γ-rays from ²⁸Al produced by the ²⁸Si (n, p) reaction. This becomes appreciable when a large amount of the sample and/or high 14 MeV neutron flux are used in order to increase the sensitivity. The relation between the saturation activity, A sub(infinity) (nfσ), and the random sum counting rate was studied. The random sum of the ²⁸Al γ-rays themselves must be taken into consideration when the A sub(infinity) value is high. The apparent half life of the random sum counts was calculated as 69.3 s, and accurate decay analyses of γ-rays were obtained by using this half life as the additional factor. The A sub(infinity) value must be kept less than 1.2 x 10⁵ s^-1 so as to avoid the fatal interference caused by the triple random sum of ²⁸Al. The external standard method was adopted. By the present method, oxygen contents were determined to be (0.06 -- 1.40)% for 20 kinds of silicon samples, 20.0% for a carbide sample and (0.94 -- 3.05)% for 13 kinds of nitride samples. (author)