Most of glaciers in Southern Chile have been retreating and shrinking during recent decades in response to atmospheric warming and decrease in precipitations. However, some glacier fluctuations are directly associated to the effusive and geothermal activity of ice-covered active volcanoes widely distributed in the region. In order to analyse these effects, an ongoing glaciological and geological research programme has been carried out on ice covered active volcanoes which have experienced a very distinctive and different recent eruptive histories. The aim of this paper is the study of the ice volumetric changes by comparison of several topographic data sets. A maximum mean ice thinning rate of 0.81 ± 0.45 m a^-1 was observed on the ash/debris covered ablation area of Volcán Villarrica between 1961 and 2004, whilst on Volcán Mocho the signal to noise ratio was not large enough to yield any conclusion. The highest percentage of glacier area reduction since 1976 was obtained on Volcán Mocho (17 %), while on Volcán Villarrica the area change rate was higher yielding 0.090 ± 0.034 km2 a^-1 between 1976 and 2005. Ice thicknesses were also measured on both volcanoes, with maximum depths of 270 m on Volcán Mocho and 195 m on Volcán Villarrica. At Volcán Villarrica the internal structure of the ice was much more complex due to the presence of intra and supra glacier layers of ash and pumices, which attenuated and obscured bottom reflections. The total ice volume present in both volcanoes yielded 4.2 ± 1.8 km³ of water equivalent (w. eq.), which is much smaller and accurate than previous estimations, improving our understanding of lahar risks in this active volcanic region. The volumetric changes experienced in recent decades by the studied glaciers yielded similar values on both volcanoes, which is unexpected considering the much more active condition of Volcán Villarrica. Glaciers on active volcanoes are therefore shrinking mainly in response to climatic driving factors. However, volcanic activity is affecting glaciers in two opposed directions; ash/debris advection is helping to reduce surface ablation at lower reaches by insulating the ice from solar radiation, while geothermal activity is probably enhancing melting and water production at the bedrock, resulting in negative ice elevation changes.