The runaway and blowdown of a non tempered hybrid chemical system (30% cumene hydroperoxide) exposed to an external heat input was investigated using a 0.1 l scale tool. The maximum temperature and the maximum temperature rise rate were showed to be sensitive to the vent size. An Antoine type correlation between the maximum temperatures and pressures was observed. These resulted from the presence of vapour, mainly generated by the reaction products. Increasing the initial filling ratio resulted in an earlier vent opening but did not have a significant influence on the blow-down. Three types of mass venting behaviour were observed, when changing the vent area to volume ratio (A/V): for large A/V, two-phase venting occurred from the vent opening until the end of the second pressure peak; for medium A/V, two-phase venting occurred before and after the turnaround. The data seem to indicate that gas only venting occurred at turn-around; for low A/V, two-phase venting was observed only after the second pressure peak.Two-phase venting after the second pressure peak probably results from the boiling of the hot reaction products at low pressure. K Ke ey yw wo or rd ds s: : Safety vent sizing; Run-away; Blowdown; Non tempered system; Cumene hydroperoxide