This paper describes a method for determining the type of organic matter in whole-rock samples through continuous detection of the carbon monoxide emitted during temperature-programmed pyrolysis. Organic matter Types I, II, and III produce distinctive carbon monoxide pyrograms. Sediments containing mixtures of these types produce pyrograms with intermediate patterns. The method is applicable to organic matter with vitrinite reflectance values up to approximately 0.75%. Analyses of natural and artificially treated samples show a similar progression in the patterns of the carbon monoxide pyrograms with increasing thermal maturity.