Absorption of tunable diode laser radiation can detect small concentrations of gases with fast response. The selectivity of the method is demonstrated by the detection of sulfuric acid in the presence of water vapor. The speed of the method is demonstrated by the first measurements of carbon monoxide (CO) concentrations in automotive exhaust with a time response (<25 ms) fast enough to be useful in the analysis of engine and emission control system dynamics. The CO was measured before and after a catalytic converter under operation at: 1) constant air/fuel (A/F) ratio, 2) step changes in A/F, and 3) oscillations of A/F at frequencies on the order of Hz.