The properties of the products of isothermal transformation of undercooled austenite into bainite in unalloyed ductile cast iron were investigated using X-ray diffraction. The following parameters were investigated: the fraction of austenite in the cast iron matrix, the crystal lattice parameter, and the width of the diffraction lines of theαandγphases. The structures were studied using a TEM. It was observed that the temperature TAand timeτAof the isothermal transformation significantly influence the nature of theαandγphases. The transformations are determined by the diffusion of carbon, and the maximum carbon content is approximately twice the equilibrium carbon content at the austenitising temperature. The lattice parameter of theαphase in the range of TAstudied decreases with increasing cooling time but increases in the upper bainite range. The increase in this lattice parameter results from the typical process of bainitic transformation during the retained austenite eutectoidal reaction (stage III). The crystal structure of theγphase in the upper bainitic region is more perfect than in the lower range. Within the investigated temperature range of TA, bainitic ferrite continually improves its crystal structure.MST/3104