The action of heteronuclear decoupling applied to I spins while observing the resonance of a 2nd nuclear species S is examd. for systems prepd. in an arbitrary initial state. Unexpected effects may be produced; the removal of heteronuclear interactions by strong decoupling fields does not guarantee that the 2 spin systems behave independently. In particular, antiphase multiplet magnetization of the S spins remains directly responsive to the radio-frequency I spin perturbation, and to I-I interactions, even when the decoupling removes the heteronuclear coupling interactions completely. Some simple illustrative examples are examd. theor. and exptl. [on SciFinder (R)]