A tertiobutylmercaptan (TBM) leak occurred on a pipeline connecting 2 storage tanks inside a plastic factory. After transferring 2 tonnes of TBM from the day tank to the site’s main supply tank, the 2 valves isolating the transfer line were closed. Heat insulated and traced in order to avoid TBM solidification at 0°C, this 2-inch diameter, 300-m long line had thus remained loaded (with 300 g) for several hours without any material flow. The thermal dilatation of TBM caused a pressure rise in the pipe not fitted with any relief valve, followed by rupture of the joint on a pallet used to insulate the inlet of an unused pipe, and then a TBM spill in the tens of litres onto the floor. While non-toxic, TBM is an irritant and extremely malodorous (with an olfactory threshold of 0.1 ppb); its exhalations could be noticed as far away as the adjacent municipality, where fire-fighters received many phone calls. The plant’s internal emergency plan was activated and its operator contained the leak through partially draining the pipe by gravity into the day tank and neutralising the spilled TBM with bleach, thus eliminating the foul smell. The defective joint was replaced. The causes of this accident were analysed and several corrective actions implemented, namely: installing a thermal dilatation valve onto the pipe, limiting the tracing temperature to 10°C, and in the interim issuing temporary guidelines to blow out the pipe before isolation. Regarding feedback from the incident, an assessment of the other site facilities demonstrated the benefit of installing relief valves on other transfer lines and acknowledged TBM thermal dilatation in the new storage tank safety report.