Thermal runaway is normally defined as the increase in charge or float current that occurs as a result of the increase in cell temperature from the initial applied constant potential. If left unchecked, the currents can reach high values and, ultimately, lead to the destruction of the cell. This definition does not explain why all cells floated at constant potential do not suffer from thermal runaway. The aim of this paper was to investigate and explain the cause of this transition from normal stable behaviour to unstable thermal runaway. A series of 6V, 100Ah, valve-regulated lead-acid (VRLA) batteries were overcharged at potentials of up to 2.65V per cell and the currents, temperatures and gas-evolution rates measured during thermal runaway. From these results, it was concluded that separator dry-out was the critical parameter that controls thermal runaway behaviour. This conclusion was reinforced by other data for the effect of saturation on the resistance, the normal float behaviour and the gas transport in VRLA separators. A model of the structure of partially saturated separators was developed to explain the observed behaviour, and was used to predict possible improvements in separator structure to increase resistance to runaway.