Results from the first measurements of core plasma poloidal rotation velocity (vθ) across Internal Transport Barriers (ITB) on JET are presented. The data was also taken during discharges with tritium puffng, providing a unique opportunity for combined studies of particle and momentum transport. The spatial and temporal evolution of the ITB can be followed along the vθ radial profiles, providing a very clear link between the location of the steepest region of the ion temperature gradient and localised spin up of vθ. The vθ measurements are an order of magnitude higher than the neoclassical predictions in the ITB region, contrary to the close agreement found between measured and predicted particle and heat transport coeffcients. These results have significant implications for the understanding of transport barrier dynamics, due to their large impact on the measured radial electric field profile.