The standard Mooney measurement has been used for more than fifty years to characterize the rheological properties of natural rubber. According to international specifications, the Mooney viscosity of a material is the torque value read after subjecting the sample to shearing for 4 minutes, at 2 rpm, in a particular geometry. In this study, the Mooney viscosity of masticated and raw natural rubber was examined by using a multiple rotor speed Mooney viscometer. The double-logarithmic flow curve shows a decrease in the slope, which is characteristic of a transition from a properflow regime to material failure, also observable by capillary rheometry. For all the samples studied, this transition takes place at rotor speeds lower than 2 rpm. These results suggest that standard Mooney measurements performed on natural rubber give misleading results. A Mooney characterization of natural rubber at lower speeds is recommended.