The composition of gas emitted from a volcano producing basalt magma can vary during an eruption and according to the volcano-tectonic setting of the degassing vents. Post-eruptive filter-pack gas samples from the 2014–2015 Holuhraun crater in the Bárðarbunga rift zone have lower ratios of S over halogens (Cl and F) and elevated F/Cl (~ 50 times lower S/Cl and ~ 5 times higher F/Cl; mass ratios) compared with samples of the syn-eruptive gas plume. The compositional changes are readily explained by Rayleigh distillation with decreasing sulphur concentrations and increasing concentrations of halogens and F relative to Cl in the final gas phase. For Cl, the vapour-melt partition coefficient (DV/M) decreased from 13–85 to 2.2 during residual degassing, whereas that of F remained uniform at approximately 1.8. Distinctly different degassing behaviour is observed for Cl and F. High D for Cl may indicate an important influence of sulphur and water on Cl volatility in basaltic melt, whereas that of F remains unaffected. The primary gas of the Holuhraun magma had similar ratios of S over Cl and F as observed at the Kilauea rift zone which, together with lower S/halogens in the residual gas in both cases, suggests similar degassing mechanism. By inference, initial CO2 degassing is likely to have occurred subglacially close to the Bárðarbunga central volcano before and during the 2014–2015 eruption on the rift-related fissure swarm.