Experimental cross-polarization transfer rates, from protons in the local dipolar field to carbons in an applied radio-frequency field, can be used to determine quantitatively the relative contributions of spin-lattice and spin-spin processes to carbon Tlp relaxation. We show that this procedure is to be preferred to determinations based either on various semiquantitative arguments, on the analysis of the Hi dependence of Tlp, or on the use of theoretically calculated rigid-lattice spin-spin transfer rates. We find that average carbon Tip's (near 35 kHz) for a wide variety of glassy polymers at room temperature are predominantly spin-lattice (motional) in character.