Oxygen and hydrogen evolution reactions in flooded lead-acid batteries during float charging were studied by galvanostatic steady-state polarization and impedance spectroscopy techniques. Given the very low relaxation frequencies of such processes (between ? and 0.05 mHz), impedance measurements needed to he extended to the ultra-low frequency domain. Investigation of their dynamic behavior provided a complete description of the porous character of the active zone. The steps involved in the reaction mechanisms are also discussed.