The hydrogen evolution and oxidation kinetics in NiMH batteries have been investigated under temperature-controlled, steady-state, overdischarging conditions within a temperature range of 10 and 50°C and at discharging currents of 1–330 mA (0.0009 to 0.3 C rate). In situ Raman spectroscopic analyses of the gas phase showed that hydrogen is the only gas evolving inside the battery during overdischarge at the above-mentioned conditions. The pressure increase could be very critical at low temperatures, leading to opening of the safety vent at relatively low discharging currents, for example, only 220 mA at 10°C. The polarization parameters for the hydrogen evolution reaction, such as Tafel slopes and exchange currents were determined at the different temperatures as well as the activation energy for the evolution and oxidation processes. The reaction mechanisms and the rate-determining steps are discussed. These are highly valuable information in NiMH modeling as they are obtained directly from the system of interest. Furthermore, the obtained results make battery simulations more realistic by minimizing the number of parameters involved and making the correct assumptions.