Intermediate species formed in the cool ignition stage of autoignition were evaluated by exhaust gas analysis with FT-IR in a test engine at hot ignition suppressed conditions. PRF (iso-octane/n-heptane) and NTF (toluene/n-heptane) were used as the fuels. The fuel consumption rate decreases with increasing iso-octane content in PRF and toluene content in NTF. HCHO generation rate increases with increasing iso-octane content in PRF but the opposite trend was found in NTF. These tendencies correspond to the difference in the detail reaction mechanism for PRF and NTF oxidation.