A much desired, direct, in situ, diagnostics technique is implemented to measure time variations of temperature and water concentration for a reactive mixture in a rapid compression machine (RCM) using quantum cascade laser absorption spectroscopy near 7.6 mu m. The temperature measurements in the RCM are successfully conducted for an end of compression pressure of P-C similar to 11.5 bar and an end of compression temperature of T-C similar to 1022 K for H-2/O-2/Ar mixtures with (0.5% by mole in the reactive mixture) and without water doping. For all cases investigated, the molar percentages of H-2 and O-2 in the reactive mixture are respectively kept at 3% while the balance is Ar, in order to modulate the extent of post-ignition pressure rise. Absorption lines of water at 1316.55 cm(-1) and 1316.97 cm(-1) are used in this study for the measurements. A six-pass setup inside the RCM is implemented for the reactive experiments. For determining the line broadening parameters of water in the range of 1316.4-1317.8 cm(-1) with H-2 or O-2 as a bath gas, a six-pass, 50.8 mm Herriott cell is used in the calibration experiments. The broadening parameters are measured in the pressure range of 300-650 Torr, while the Herriott cell is heated uniformly to temperatures of 460, 500, and 530K, respectively. With the measured line broadening parameters of water in the bath gases of interest, the temperature and water concentration histories in the RCM runs are experimentally determined. In addition, the experimental results are compared with the simulations with detailed chemistry. Reasonable agreement is found, thereby demonstrating the utility of this mid-IR absorption spectroscopy in RCM experiments. The uncertainties of the associated measurements of temperature and water concentration are also discussed. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.