The mineral apatite can incorporate all of the major magmatic volatile species into its structure. Where melt inclusions are not available, magmatic apatite may therefore represent an opportunity to quantify volatile concentrations in the pre-eruptive melt. We analysed apatites and matrix glasses from andesites and dacites erupted from Santiaguito Volcano, Guatemala, between the 1920s and 2002. X-ray mapping shows complex zoning of sulphur in the apatite grains, but typically with sulphur-rich cores and sulphur-poor rims. Apatite microphenocrysts are enriched in F and depleted in Cl relative to inclusions. Matrix glasses are dacite to rhyolite and contain low F but up to 2400 ppm Cl. Overall, the data are consistent with progressive depletion of Cl in the most evolved melts due to crystallisation and degassing. In the absence of pristine melt inclusions, we used apatite, together with published partitioning data, to reconstruct the likely volatile contents of the pre-eruptive melt, and hence estimate long-term average gas emissions of SO2, HF and HCl for the ongoing eruption. The data indicate time-averaged SO2 emissions of up to 157 tonnes/day, HCl of 74-1382 tonnes/day and up to 196 tonnes/day HF. Apatite may provide a useful measure of long-term volatile emissions at volcanoes where direct emissions measurements are unavailable, or for comparison with intermittent gas sampling methods. However, significant uncertainty remains regarding volatile distribution coefficients for apatite, and their variations with temperature and pressure.