Ultra-violet absorption and infra-red emission techniques have been used to follow the vibrational relaxation of nitric oxide after shock heating in a temperature range from 450°K to 1300°K. The infra-red technique yields a vibrational de-excitation probability, P 10, an order of magnitude or more below the ultra-violet results. A simultaneous experiment has also been performed with both methods at the same time, and it is found that the disagreement persists. Therefore, if the infra-red results represent the vibrational relaxation, then the ultra-violet light source has no effect. Possible interpretations of the experiments are offered and discussed with the purpose of removing the discrepancy. Suggestions for further experiments are also set forth.