A procedure is demonstrated for determining the relative contributions of spin–lattice and spin–spin interactions to T1ρ(C), the relaxation time characterizing the loss of spin-locked carbon polarization. This involves the characterization of relaxation processes associated with several cross-polarization experiments. Three glassy polymer T1ρ(C) values are shown to be dominated by spin–lattice effects while a crystalline polymer relaxes through a spin–spin mechanism.