Studies on the kinetics of gas-phase chemical reactions currently rely on calculations or simulations and lack simple, fast, and accurate direct measurement methods. We developed a tunable laser molecular absorption spectroscopy measurement system to achieve direct measurements of such reactions by using wavelength modulated spectroscopy and performed online measurements and diagnostics of molecular concentration, reaction temperature, and pressure change during the redox reaction of ozone with nitrogen oxides (NO_x) with 0.1 s temporal resolution. This study provides a promising diagnostic tool for studying gas-phase chemical reaction kinetics.