Abstract. The oceans are a key source of a number of atmospherically important volatile gases. The accurate and robust determination of trace gases in seawater is a significant analytical challenge, requiring reproducible and ideally automated sample handling, a high efficiency of seawater–air transfer, removal of water vapour from the sample stream, and high sensitivity and selectivity of the analysis. Here we describe a system that was developed for the fully automated analysis of dissolved very short-lived halogenated species (VSLS) sampled from an under-way seawater supply. The system can also be used for semi-automated batch sampling from Niskin bottles filled during CTD (conductivity, temperature, depth) profiles. The essential components comprise a bespoke, automated purge and trap (AutoP & T) unit coupled to a commercial thermal desorption and gas chromatograph mass spectrometer (TD-GC-MS). The AutoP & T system has completed five research cruises, from the tropics to the poles, and collected over 2500 oceanic samples to date. It is able to quantify >25 species over a boiling point range of 34–180 °C with Henry's law coefficients of 0.018 and greater (CH₂2l, k_H^cc dimensionless gas/aqueous) and has been used to measure organic sulfurs, hydrocarbons, halocarbons and terpenes. In the eastern tropical Pacific, the high sensitivity and sampling frequency provided new information regarding the distribution of VSLS, including novel measurements of a photolytically driven diurnal cycle of CH₂2l within the surface ocean water.