We develop the connection between theoretical studies of so-called quasi-equilibrium states in solid-state NMR for isolated spin-systems with the experimental observability of such states. The observability of these states is expected to be dependent on the relative broadenings due to coherent effects (such as dipolar couplings with the lattice) and incoherent effects, i.e. relaxation. Hence, we expect to see evidence of quasi-equilibria in cross polarization experiments, where the relevant relaxation time constant is relatively long. For classic spectral spin-diffusion experiments, however, quasi equilibria are unlikely to be observed. We also generalise the discussion to rotating solids, which is appropriate to magic angle spinning experiments. (C) 1998 Elsevier Science B,V. All rights reserved.