Side reactions that lead to gas evolution are undesirable in batteries, and result in reduced Coulombic efficiency and shortened lifetime. Quantitative analysis of the gases that develop istherefore important to increase the understanding of the reactions occurring in the batteryduring cycling and could be used to optimize battery operation. However, the measurementsare associated with challenges because batteries are by their nature closed with limited gasspace. Nickel metal hydride (NiMH) batteries are widespread due to good rate capability,reliability, and environmental friendliness. Although the battery type is thoroughlyinvestigated, studies of battery gas composition formed during a work cycle are few. In thisstudy, two methods for investigating the internal NiMH battery gas phase composition duringdifferent charge/discharge cycles using a mass spectrometer (MS) were developed. In the firstmethod, the battery module was connected by a sampler system. In the second method, thebattery was connected directly using a microcapillary, and the gas composition wascontinuously measured. In addition to the gas composition the voltage, pressure, andtemperature of the battery were recorded. The biggest contributor in the measured gas phasewas nitrogen, present in the cell already from the assembling, followed by hydrogen. A clearrising trend of hydrogen pressure as depth of charge (DOC) increased was recorded, whileoxygen levels were low except around end of charge. The methods were found to be a reliableway of investigating NiMH gas composition without negatively affecting the battery and maybe adapted to other battery chemistries.