The interpretation of the detected onset temperature for DSC and other calorimetric measurements used for Reactive Chemicals is briefly discussed in terms of calorimeter sensitivity and reaction dynamics. Common misinterpretations are also addressed. It is shown that the detected onset temperature should not be considered as an upper stability limit for a process defining the 'start' of undesired reactions. It is also not a point below which processes can operate safely at any temperature, In the latter case, an arbitrary choice of operating somewhere below a detected onset temperature can stall desired chemistry, lead to an over-conservative safety margin impacting cycle time/production rates, or provide a false sense of security triggering serious consequences. An understanding of the heat gain rates and heat loss rates is necessary to engineer for safe operations. (C) 2002 Elsevier Science Ltd. All rights reserved.