A study of carbon monoxide (CO) dissociation was performed in a shock tube at conditions relevant to the high-speed entry of Venus and Mars atmospheres. The CO number density (or mole fraction) and the temperature are probed behind reflected shock waves at 1 MHz using scanned-wavelength laser absorption spectroscopy near [Formula: see text] ([Formula: see text]). The wide range of vibrational states ([Formula: see text], and 10) probed by this technique enables precise number density and temperature measurements up to and above 9000 K using a Boltzmann population fit of the resolved spectral lines. Mixtures of CO diluted in Ar at 3–60% are shock-heated in a wide range of conditions ([Formula: see text] at [Formula: see text]) and compared to state-of-the-art chemical kinetic models. The time-resolved measurements of temperature and number density behind reflected shock waves are utilized to infer the rate coefficients of [Formula: see text] for [Formula: see text], Ar. They are found to be [Formula: see text] and [Formula: see text].