In this report, samples of pure iron were carbonitrided under various conditions by using C₂H₆,NH₃, and H₂ gas mixtures, and the changes near the starting temperature of austenitization were measured by means of differential thermal dilatation, X-ray diff action, micro-Vicker’s hardness, and chemical analysis tests. The results in summary were as follows : (1) The double-staged contraction beginning at 590∼605° and 620∼650° is du to γ-generation. The transition b gins with the first stage. (2) Layers of compounds of high-N low-C ε+γ′ and high-N high-C θ+ε+γ′ were found, usually in large thich ess. (3) The cause of lowered surfacial hardness and scattering of the measured values in the hardness distribution curve of anncealed specimens lies mainly in the existence of irregular black spots formed by change in concentration during cooling. (4) A radical revision of the experimental method was felt desirable for assuring better accuracy.