In this work, we report the measurement of the viscosity of biodiesels derived from waste cooking oil and vegetable oil under high pressure. Using a falling sinker-type viscometer, dynamic viscosity measurements were made at pressures up to 140MPa which are typically to be found in common-rail automotive diesel engines. Reproducible and reliable viscosity data were obtained from sinker fall times and calibration data. The biodiesel viscosities were compared to that of petrochemical diesel fuels also under high pressure. Both fuel types were found to increase exponentially with pressure with the biodiesels being slightly the more viscous. Above 140 MPa, a marked increase in sinker fall time was observed. This was due to a pressure-freezing effect which is known to also occur with longer chain molecules found in petrochemical diesel, oils, fatty acids and biodiesel. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.