The thermal decomposition of ethanol has been studied under pyrolytic conditions behind reflected shock waves in the 1250 to 1677 K temperature range, at an average pressure of 1.31 atm for a mixture highly diluted in Ar. A laser absorption technique was utilized to measure H₂O time-histories, and the detailed kinetics mechanism (AramcoMech2.0) was selected among various models from the literature based on its a priori agreement with the experimental data in the present study. Sensitivity and rate-of-production analyses were performed and showed that the C₂H₅OH→C₂H₄+ H₂O (R1) decomposition pathway is almost the sole reaction contributing to H₂O formation at the early times under the present conditions, allowing an a priori direct measurement of its rate coefficient k₁. The rate coefficient was determined to be defined as the Arrhenius equation k₁ (s⁻¹) = 3.37 × 10¹¹ exp (–27174 K/T), which is in very good agreement with Kiecherer et al. (2015), where k₁ was also directly determined under similar conditions. Secondary chemical reactions taking place in the thermal decomposition have very low influence in the H₂O formation during the time-frame selected, leading to an uncertainty for k₁ of approximately 20%. The full H₂O time histories are useful for validating the full ethanol kinetics mechanism for future validation.