Sulfur speciation in volcanic gases is a potentially valuable tracer of degassing processes at volcanoes. Hitherto, observations of sulfur speciation in volcanic gas plumes have however been limited both in number and quality. Here, we report on periodic measurements of SO2 to H2S proportions in the volcanic gases from La Fossa volcano (Vulcano Island) performed during 2004--2006, a period which encompasses two heating events of the fumarolic field in January--April 2005 and December 2005. Results indicate a systematic relative increase (by a factor of 2--6) of SO2 to H2S proportions in the fumaroles during the heating events, which we ascribe to a temperature increase in the mixing zone between magmatic and hydrothermal fluids. We also demonstrate that sulphur speciation in La Fossa fumaroles reflects re-equilibration within a poly-baric hydrothermal system, and that this hydrothermal re-equilibration erases the pristine SO2/H2S ratios of any magma-derived sulphur present.