SiF4 is probably a common trace component of fumarole gases on volcanoes, arising from interactions between magmatic HF and wall rocks. It is not distinguished from HF in conventional analyses. A strong absorption band at 1032 cm-1 makes SiF4 well suited to remote open-path FTIR spectroscopic techniques. We report the first measurements of SiF4 in volcanic gases at Etna and Vulcano (Sicily). SiF4 concentrations were low at Etna, but higher at Vulcano, where the SO2:SiF4 ratio was consistently around 1.7¿10-2:1. Thermodynamic considerations show that the HF:SiF4 ratio increases sharply with increasing temperature. Thus, remote measurements of SiF4 may offer a qualitative way of constraining fumarole temperatures. We use our FTIR measurements of HCl:SiF4 in combination with conventional measurements of HCl:HF to infer an HF:SiF4 ratio for Vulcano of ~10:1, consistent with gas temperatures in the range 200--450 ¿C. ¿ American Geophysical Union 1996