Results from the first measurements of a core plasma poloidal rotation velocity (v
) across internal transport barriers (ITB) on JET are presented. The spatial and temporal evolution of the ITB can be followed along with the v_? radial profiles, providing a very clear link between the location of the steepest region of the ion temperature gradient and localized spin-up of v_?. The v_? measurements are an order of magnitude higher than the neoclassical predictions for thermal particles in the ITB region, contrary to the close agreement found between the determined and predicted particle and heat transport coefficients [K.- D. Zastrow et al., Plasma Phys. Controlled Fusion 46, B255 (2004)]. These results have significant implications for the understanding of transport barrier dynamics due to their large impact on the measured radial electric field profile.