An electron microscopy study is made of the features of formation and evolution of the structure of steel 90G28Yu9VBM during ageing in the temperature range 350-1000°C. The sequence and character of structural changes at all stages of decomposition of the supersaturated austenite are determined. A detailed examination is made of the morphological types and parameters of the modulated structure that arises during homogeneous nucleation of K-phase, and of the structural parameters of the decomposition products on the boundaries of austenite grains. The temperature-time conditions of ageing of the steel for which either homogeneous, isostructural decomposition of the γ-solid solution or cellular decomposition take place are determined and the combined effect of the two mechanisms is also observed.