A new method for early detection of runaway reactions in dosed vessels is proposed. The method is based on continuous monitoring of the gas phase composition using on-line mass spectrometry, and can be used in reactions yielding gaseous products. The method has been applied to the study of the decomposition of organic peroxides (dicumyl peroxide and di-tert-butyl peroxide) in a RSST unit, and found to be faster and more reliable than traditional methods based on monitoring the temperature of the liquid phase.