Lahars (volcanic debris flows) have been responsible for 40% of all volcanic fatalities over the past century. Mount Semeru (East Java, Indonesia) is a persistently active composite volcano that threatens approximately one million people with its lahars and pyroclastic flows. Despite their regularity, the behaviour and the propagation of these rain-triggered lahars are poorly understood. In situ samples were taken from lahars in motion at two sites in the Curah Lengkong River, on the southeast flank of Semeru, providing estimates of the particle concentration, grain size spectrum, grain density and composition. This enables us to identify flow sediment from three categories of lahars: (a) hyperconcentrated flow, (b) non-cohesive, clast- and matrix-supported debris flow, and (c) muddy flood. To understand hyperconcentrated flow sediment transport processes, it is more appropriate to sample the active flows than the post-event lahar deposits because in situ sampling retains the full spectrum of the grain-size distribution. Rheometrical tests on materials sampled from moving hyperconcentrated flows were carried out using a laboratory vane rheometer. Despite technical difficulties, results obtained on the <63, <180, and <400 mu m fractions of the sampled sediment, suggest a purely frictional behaviour. Importantly, and contrary to previous experiments conducted with monodisperse suspensions, our results do not show any transition towards a viscous behaviour for high shear rates. These data provide important constraints for future physical and numerical modelling of lahar flows. Copyright (C) 2010 John Wiley & Sons, Ltd.