The study deals with the light absorption and emission behavior in the preflame reaction interval before hot flame reactions.(1-3) Absorption spectroscopy was used to measure the behavior of HCHO and OH radicals during a progression from normal combustion to knocking operation. Emission spectroscopic measurements were obtained in the same way that radical added HCO. Radical behavior in preflame reactions was thus examined on the basis of simultaneous measurements, which combined each absorption wavelength with three emission wavelength by using a monochromator and a newly developed polychromator.(4-5)When n-heptane (0 RON) and blended fuel (50 RON) were used as test fuel, it was observed that radical behavior differed between normal combustion and knocking operation and a duration of the preflame reaction was shorter during the progression from normal combustion to a condition of knocking.The measurements obtained for HCHO when n-heptane was used as the test fuel showed an increase in emission intensity in the early period of the preflame reactions under normal combustion, followed by a decline in intensity. Under a condition of knock induced by an overheated cylinder head, it was observed the maximum emission intensity decreased. These tendencies are thought to correspond to the passage and degeneracy of a cool flame.As blended fuel (50 RON) was test fuel, the measurements obtained that behavior thought to indicate the passage of a cool flame, but the characteristics associated with the degeneracy of a cool flame were not observed. Moreover a result was found that the behavior of HCO is more important factor in the mechanism causing knock than the behavior of HCHO.