A series of 50 wt% Pt/C catalysts based on carbon blacks with different specific surface areas and morphologies was synthesized and investigated using "start-stop" protocol (triangular scan in the 1.0-1.5 V vs. reversible hydrogen electrode (RHE) potential range with a scan rate of 500 mV s-1). The commercial 40wt%Pt/VulcanXC-72 and 50wt%Pt/TEC were used for comparison. The oxygen reduction reaction (ORR) activities and electrochemically active surface area (ECSA) of Pt were obtained at 25 °C in 0.1 M HClO4 electrolyte after every 4000 cycles. All synthesized Pt/C catalysts were characterized by X-ray powder diffraction (XRD), highresolution transmission electron microscopy (HRTEM), low-temperature nitrogen sorption, CO pulse chemisorption methods, and cycling voltammetry including a rotating disc electrode (RDE) method for analysis of the ORR kinetics. Dependences of the ORR activities, ECSA, and values of Tafel slopes on the number of oxidative cycles were obtained and analyzed in detail. It was shown that the initial values of Tafel slopes of the Pt/C catalysts decrease with increase of the surface area of carbon blacks. The increase in the values of Tafel slopes in the high-current density region with cycling was associated with the degradation of carbon support. The reciprocal ratios of the ECSA to initial values were found to grow linearly on the number of cycles. The ORR surface activity was significantly decreased in the first 8000-10, 000 oxidation cycles as the result of changes of the active component and carbon support degradation. The following slight increase in the ORR surface activities with cycling was related to the increase in the Pt particle sizes in the Pt/C catalysts. Modification of Ketjen black EC 600 DJ through pyrocarbon deposition (KB600-C sample) led to appreciable increase of the stability of the Pt/KB600-C sample.