Abstract. Measurements for the viscosity of cyclopentane in the liquid phase are presented. The measurements cover the temperature range from 210 to 310 K, from atmospheric pressure up to 25 MPa, and were performed in a vibrating-wire viscometer. The uncertainty of the measurements is estimated to be 0.5%, rising to 1 % at low temperatures, an estimate confirmed by the comparison of the viscosity of water and toluene with standard correlations. 