Lava flows into the sea at Kīlauea Volcano, Hawaiʻi, and generates an airborne gas and aerosol plume. Water (H₂O), hydrogen chloride (HCl), carbon dioxide (CO₂), nitrogen dioxide (NO₂) and sulphur dioxide (SO₂) gases were quantified in the plume in 2004–2005, using Open Path Fourier Transform infra-red Spectroscopy. The molar abundances of these species and thermodynamic modelling are used to discuss their generation. The range in molar HCl / H₂O confirms that HCl is generated when seawater is boiled dry and magnesium salts are hydrolysed (as proposed by [T.M. Gerlach, J.L. Krumhansl, R.O. Fournier, J. Kjargaard, Acid rain from the heating and evaporation of seawater by molten lava: a new volcanic hazard, EOS (Trans. Am. Geophys. Un.) 70 (1989) 1421–1422]), in contrast to models of Na-metasomatism. Airborne droplets of boiled seawater brine form nucleii for subsequent H₂O and HCl condensation, which acidifies the droplets and liberates CO₂ gas from bicarbonate and carbonate. NO₂ is derived from the thermal decomposition of nitrates in coastal seawater, which takes place as the lava heats droplets of boiled seawater brine to 350–400 °C. SO₂ is derived from the degassing of subaerial lava flows on the coastal plain. The calculated mass flux of HCl from a moderate-sized ocean entry significantly increases the total HCl emission at Kīlauea (including magmatic sources) and is comparable to industrial HCl emitters in the United States. For larger lava ocean entries, the flux of HCl will cause intense local environmental hazards, such as high localised HCl concentrations and acid rain.