Measurement of electrochemical surface area (ECSA) of fuel cell electrodes is a key diagnostic of performance and gives a useful parameter for monitoring degradation and state of health in polymer electrolyte membrane fuel cells (PEMFCs). However, conventional methods for determining ECSA require potentiostatic control of the cell, which is impractical in a fuel cell stack. Here we demonstrate for the first time the practical application of a galvanostatic technique that enables in situ monitoring of ECSA in each cell throughout the lifetime of a stack. The concept is demonstrated at single cell level using both H adsorption and CO stripping, and the H adsorption (cathodic current) method is extended to stack testing. The undesirable effects of H2 crossover on the measurement may be minimised by appropriate selection of current density and by working with dilute H2 on the anode electrode. Good agreement is achieved with ECSA values determined using conventional single cell voltammetry across a range of MEA designs. The technique is straightforward to implement and provides an invaluable tool for state of health monitoring during PEMFC stack lifetime studies.