The knowledge of performance characteristics of turbine stage groups is still insufficient, particularly in the general case of changes of operating conditions. This situation is caused mainly by the scarcity of experimental data available. In such case, the opportunity to obtain the required data, using mathematical modelling and numerical simulation of the operation of stage groups under off-design conditions instead of physical experiment, seems to be attractive. The application of this idea for impulse type turbine stage groups was presented in [I], [2]. Here we discuss similar results but obtained for reaction type turbine stage groups, that is: - mathematical model for computer simulation of operation of reaction type turbine stage group, under variable regime (based on Ainley's and Mathieson's method with some improvements); - simulation results for a number of stage groups designed according to former BBC and traditional concepts; - more general properties of these groups (in relation to flow capacity and efficiency) obtained from the analysis of simulation data; - comparison of observed properties of impulse and reaction typy turbine stage groups.