We tried to determine the process of transformation within isothermal conditions first of all by dilatometric measurements. The aim of our examinations was to study how the characteristics of dilatometric curves changed during austempering after the austenitization carried out at different temperatures. In the course of experiments the transformation processes were followed by a self-built dilatometer. It has been stated and supported by theoretical calculations that after the volumetric change caused by the formation of bainitic ferrite, depending on the circumstances (temperature of austenitization, temperature of isotherm) the corporate formation of ferrite and carbide might cause increase or decrease in volume. Besides the dilatometric measurements, X-ray diffraction, light- and electron microscope investigations were also carried out.