A methodology describing the sampling and analysis of biogenic volatile organic compounds (VOCs) in ambient coastal air is presented. Air samples were collected during a field campaign (September 1998) at Mace Head on the west coast of Ireland. In addition, air samples from seaweed enclosures were taken to assess the VOC source potential of macro-algae at low tide. VOC identification was confirmed using GC–MS and an n-alkane retention index, with quantification carried out using GC–FID. A suite of compounds (C8–C16) were routinely quantified; these included a series of n-alkanes (C11–C16) and oxygenated hydrocarbons (C8–C14). Their mixing ratios in coastal air ranged from <10–400 parts per trillion (pptv). The most abundant oxygenates included 2-ethyl-1-hexanol (mean=175 pptv) and two ubiquitous ketones (geranyl acetone and 6-methyl-5-hepten-2-one) with average mixing ratios of 123 and 28 pptv, respectively. Air mass back trajectories indicated higher levels of the oxygenates when air flow was off the land (easterly continental) rather than from the ocean, although the opposite was observed for the n-alkanes. Mixing ratios of the n-alkanes (>C12) were significantly higher in the seaweed enclosure air than in ambient air, suggesting seaweed to be a source of long chain n-alkanes. Oxygenates, however, did not show a significant difference between enclosure and ambient air. Unlike studies carried out in the vicinity of woodlands, no monoterpenes or sesquiterpenes were detected in this study.