In the oxygen cycle of valve-regulated lead-acid (VRLA) batteries, there are two ways in which oxygen can move from the positive to the negative plates, namely, either horizontally to penetrate the absorptive glass mat (AGM) separator, and/or transport vertically via the gas space. It is found that the oxygen transport depends on the passageway with big void space in the AGM separator and its rate is proportional to the oxygen partial pressure. The rate constant of vertical transport is about three orders higher than that of horizontal transport because of the large void space between the AGM separator and plates. However, in the horizontal direction, the area is very large and the transport path is very short. So the way and the rate of oxygen transport actually depend on the level of saturation in VRLA batteries. The horizontal transport is dominant when the saturation is less than 93%, while the vertical transport becomes dominant when it is higher than 93%. The experiments also indicate that with decreasing saturation, the recombination of more oxygen at the negative plate may oxidize more active H-ad atoms and therefore, the overpotential of hydrogen evolution increases obviously.