The influence of H2SO4 solution concentration on the corrosion behavior of as-cast Al0.1CoCrFeNi high-entropy alloys (HEAs) is investigated. The potentiodynamic polarization curves of the Al0.1CoCrFeNi HEA exhibit excellent corrosion resistance and a wide passive region (ΔE > 1150 mV) in acid solutions. The samples show a spontaneous passivation behavior in low-concentration (0.1, 0.2 and 0.3 M) and two anodic active–passive peaks in high-concentration (0.5 and 1.0 M) H2SO4 solutions. The morphology investigation by scanning electron microscopy suggests that selective corrosion is initiated from the inter-dendrite regions. The X-ray photoelectron spectroscopy results demonstrate that the observed passive behavior variation is associated with chemical composition change of the passive films.