As atmospheric humidity increases, the emission of NO from automobiles decreases. This effect is so large that a factor for correcting observed NO to some standard humidity is desirable. Emission tests were conducted at 76 F and at four humidities on a fleet of eight passenger cars, in accordance with the Federal Test Cycle. A derived equation relates the concentration of NO (measured by continuous trace analysis in accordance with the proposed 1971 California procedure) to differences in air moisture at the time of the test. Factors are also included to correct NO concentrations measured during other types of operation and to correct hydrocarbon and carbon monoxide emissions measured during a Federal Cycle.