The heteronuclear cross-polarization efficiency in the rotating frame can be improved by sweeping the radiofrequency amplitude adiabatically through the Hartmann-Hahn condition. The enhancement is particularly significant for strongly coupled IS spin systems with weak dipolar coupling to extraneous spins. A good approxn. to an adiabatic transfer is obtained with a tangential time dependence of the radiofrequency field amplitude whereby transient oscillations are efficiently suppressed. [on SciFinder (R)]