Water and glycerine solutions were used to produce nominal liquid viscosities of 1, 50, 200 and 550 mPa s. These liquids flowed concurrently with air to allow measurements of momentum flux, void fraction and pressure distributions to be made at mass fluxes consistent with an exit "homogeneous" Mach number of 0.4. The flow pattern in the pipe was, in the main, annular up-flow. The data are shown to be consistent with the annular flow model. However, entrained liquid fractions were estimated at conditions where they were unexpected and the interfacial friction factors were significantly different to those implied from low viscosity fluid correlations. A new correlation for the interfacial friction factor is proposed for the highly viscous dataset. Choking tests were carried out with air-liquid flows with liquid viscosities of 1 and 50 mPa s. These data showed that the annular model approach extends to cover these flow conditions. More traditional methods are shown not to represent the measured void fraction, frictional pressure drop or choking mass flow rates. (C) 2003 Elsevier Ltd. All rights reserved.