Union Glacier, West Antarctica, was intensively mapped in December 2008, when an over-snow traverse was conducted by CECS and ALE, with the aim to determine the ice-dynamical characteristics of the glacier, through mapping the crevasse fields and by providing a glaciological baseline for future studies. A mean ice thickness of 1450 m was measured, confirming the presence of a deep subglacial topography (∼900 m below sea level), much deeper than previously estimated. Ice velocities were also measured at 21 stakes drilled into the ice at the narrowest gate of the glacier between December 2007 and December 2008, yielding a mean value of 22.6 m a−1. These velocities, combined with the measured ice thicknesses and a numerical model, yielded an ice flux of 0.10 ±0.03 km3 a−1 w.e. Considering the ice basin above this gate, a mean surface mass balance of 0.18 ± 0.05 m a−1 was estimated, a value consistent with a mean snow accumulation for nearby ice streams. These values indicate that the glacier is at present near equilibrium.