Substantial losses of plasma stored energy and toroidal ion momentum are observed in JET during large amplitude oscillating or quasi-stationary MHD activity when mode coupling effects become important. The degradation in the diamagnetic stored energy due to low m,n MHD modes increases with amplitude, reaching ΔW/W > 30% at a mode amplitude of r/Bθ > 0.4%. Favourable comparisons are made with the degradation in the incremental energy confinement time during such MHD activity as predicted by Chang and Callen. The reduction in the plasma ion toroidal momentum, from charge exchange measurements on C 6+ ions, depends on the extent of mode coupling within the plasma and on the oscillation frequency of the n = 1 mode. When r/Bθ > 0.1% for more than about 300 ms, toroidal coupling between low m,n modes together with coupling of the plasma ions to the modes by a force equilibrates the toroidal ion rotation frequency with the MHD oscillation frequency over substantial regions of the plasma, depending on the radius of the rational q surface of the coupled MHD mode. This ion mode coupling force becomes particularly apparent when the mode frequency drops to nearly zero and the ion toroidal rotation frequency also drops to zero within 100–300 ms, despite continued neutral beam injection. In such cases, the toroidal ion momentum appears to be lost electromagnetically via the MHD modes to the external structure or to fixed stray fields of the tokamak, while the plasma stored energy losses must be accounted for by other processes.