Most models for magma flow in volcanic conduits during explosive eruptions assume isothermal ascent conditions due to the high heat capacity of the magma. Here we present a non-isothermal axisymmetrical flow model that accounts for the temperature-dependent viscosity of magma and viscous dissipation of heat. Significant changes in velocity and temperature profiles result in strong reduction of the conduit friction leading to an order of magnitude increase in discharge rate in comparison with the isothermal case. This allows high intensity eruptions to occur from significantly narrower volcanic conduits and so helps to resolve inconsistencies between conduit dimensions inferred from models and observations. For a given conduit diameter fragmentation can occur at much shallow depths than for an isothermal model.