RDE-based accelerated stress tests (ASTs) are often used for early-stage research of fuel cell catalysts, in which the electrochemical surface area (ECSA) and the mass activity (MA) are usually measured only at the beginning and the end of the testing procedure, although their intermediate values in the transitional period are very important to understand the degradation mechanisms. This is because the ECSA and the MA are measured in the O-2-free and the O-2-saturated electrolytes, respectively, and frequent gas switching introduces extra burden to the already lengthy AST. In this study, we developed a new method to measure the ECSA in the O-2-saturated electrolyte, which leads to an improved AST that can acquire the intermediate ECSA and MA values without changing the purging gases. Then, we evaluated Pt/C and Pt-3 Ni/C model catalysts with the new AST. The results showed a clear difference in degradation behavior: while the MA of the Pt/C gradually reduced as a result of the ECSA loss, the MA of the Pt-3 Ni/C rapidly decreased owing to the decrease in the specific activity. This new AST protocol would enable us to accurately compare the durability of different catalysts and to deeply understand their degradation mechanism. (C) The Author(s) 2019. Published by ECS.