Co-current central impurity toroidal rotation has been observed in Alcator C-Modplasmas with on-axis ICRF heating. The rotation velocity increases with plasma stored energy and decreases with plasma current. Very similar behaviour has been seen during ohmic H modes, which suggests that the rotation,generated in the absence of an external momentum source, is not mainly an ICRF effect. A scan of ICRF resonance location acrossthe plasma has been performed in order to investigate possible influences onthe toroidal rotation. With a slight reduction of toroidal magnetic field from 4.7 to 4.5 T and a corresponding shift of the ICRF resonance from r/a = -0.36 to -0.48, the central toroidal rotation significantly decreasedtogether with the formation of an internal transport barrier (ITB).During the ITB phase, electrons and impurities peaked continuouslyfor |r/a| ⩽ 0.5.Comparison of the observed rotation and neoclassical predictions indicates thatthe core radial electric field changes from positive to negative during theITB phase. Similar rotation suppression and ITB formation have been observed during some ohmic H mode discharges.