The existence of self-heating accompanying hydrogen oxidation in closed vessels in the vicinity of the P–Ta second explosion limit has been demonstrated experimentally, apparently for the first time. Direct measurements of the extents of temperature rises in equimolar (H2+ O2) and stoichiometric (2H2+ O2) mixtures have been made using a fine-wire thermocouple-junction placed at the centre of the reaction vessel. Two cylindrical quartz vessels have been used; their volumes were 330 and 100 cm3, respectively. Self-heating during slow reaction close to the second limit is negligible only at the lowest temperatures; it becomes significant as the vessel temperature is increased. The temperature rise exceeds 3 K at locations in the P–Ta diagram close to where the second and third limits coalesce. Still greater extents of self-heating are measured in the slow reaction zone at pressures higher than those at the second limit. This study offers the first experimental test of the numerical predictions using chain-thermal theory: encouraging agreement is found.