We investigate sulphur, chlorine and fluorine release during explosive, effusive and intrusive phases the 2011- 2012 Cordón Caulle eruption, with a focus on the halogens. Analysis of melt inclusions, pyroclasts and lava samples shows most sulphur to have degassed during magma decompression, but halogen release to have accompanied isobaric crystallisation in slowly-cooled magma that was emplaced in a lava flow and sub-vent intrusion. Fluorine in particular mobilised only after extensive groundmass crystallisation and incipient devitrification. By 2017, gas emitted from vent-proximal fumaroles had hydrothermal compositions, with HCl/HF ratios correlating with temperature. We estimate that the eruption could eventually emit up to 0.92 Mt of SO2, 6.3 Mt of HCl, and 1.9 Mt of HF, but only 16 wt.%, 7 wt.% and 2 wt.% of these were respectively emitted during opening explosive phases. Halogen devolatilisation and its associated hazards can persist long after rhyolite eruption and/or emplacement.