Decarburization of hypoeutectoid carbon steels during isothermal annealing in air at temperatures Ac1 < T < AC3 and different annealing times was researched. Aforementioned steels have a dual-phased ferrite-austenite microstructure at these temperatures. The effects of annealing temperatures and times on the depth of decarburization of C22, C45 and C60 steels were researched. Decarburization of samples was primarily researched metallographically. Also the EDS analysis was made, to determine the effects of segregations of certain alloying elements on the kinetics of decarburization. Theoretical calculation of the depth of decarburization was made by Van-Ostrand-Dewey equation. It was found out that this equation is also suitable for calculating the decarburization at AC1 < Tž < AC3 even though it was primarily meant for the decarburization of austenite at T > TG. The lowest known value of the diffusion coefficient of carbon needs to be used with this equation. Equations, that can be used for calculation of the theoretical depths of decarburization for experimental conditions used in this thesis, were created from Van-Ostrand-Dewey equation. The comparison between measured and calculated results shows high levels of matching.