An experimental study of flow boiling heat transfer to a highly viscous fluid in vertical downward flow is presented for a consistency index eta(0) = 0.71-26.66 Pa s(n), tube diameters D = 5.34, 7.71 and 10.27 mm, mass velocity G = 4.36-53.4 kg m(-2) s(-1), a quality X = 0.0-0.54 and a heat flux q = 1.43-14.35 kW m(-2). A correction based on the present experimental data has been developed which takes account of the effect of liquid viscosity on flow boiling heat transfer.