A fast temperature sensor based on intrapulse absorption has been applied to cases of two-stage ignition of iso-octane in a rapid compression machine. Two pulsed quantum cascade lasers at 5.46 and 5.60 µm respectively were used to record the spectra of two separate rovibrational lines of H2O using intrapulse downchirp which resulted in a large tuning range (1.8 - 2.3 cm-1). The temperature was determined form the measured spectra using a calibration-free two-line thermometry method. A pulse repetition rate of 100 kHz was used for high temporal resolution in temperature measurements. Experiments were performed using stoichiometric iso-octane/air mixtures for EOC conditions of 15 - 20 bar and 671 - 735 K which correspond to the NTC and low temperature region where two-stage ignition can be observed. The temperature rise during first stage heat release was quantified and compared with kinetic simulations using the LLNL iso-octane model.