An in-situ temperature diagnostic based on intra-pulse absorption spectroscopy has been developed using two pulsed quantum cascade lasers (QCLs) centered at 5.46 and 5.60 μm for rapid compression machine (RCM) experiments. Pulsed mode operation of the QCLs yielded a broad spectral tuning range (1.8–2.3 cm−1), through which spectral line-shapes of two H2O ro-vibrational transitions were resolved at high pressure conditions in the RCM (15–20 bar). Based on the resolved line-shapes, a calibration-free two-line thermometry method was used to determine the gas temperature. A high temporal resolution of 10 μs was achieved through a pulse repetition frequency of 100 kHz. The diagnostic was validated through measurements of temperature rise during the first-stage ignition of n-pentane/air mixtures. Thereafter, temperature rise during the first-stage ignition of iso-octane/air mixtures was quantified for the first time and compared with the calculated temperature rise using a chemical kinetic model.