The effects of different factors such as the state of charge (SOC), the temperature, the polarization potential, the H2SO4 concentration, and the saturation level of the negative plate on oxygen recombination in valve-regulated lead-acid batteries were investigated. The increase of the temperature and of the negative polarization as well as the decrease of the saturation level of the negative plate can all promote oxygen recombination, yet the SOC has almost no effect on the oxygen recombination as long as the capacity of the negative plate is high enough. Furthermore, it is found that the oxygen recombination rate increases and then drops when the H2SO4 concentration at the negative plate increases from 0 to 5 M. The experiments also show that considering all factors, the saturation level of the negative plate is the dominant factor affecting the oxygen recombination rate.