The P-31 solid-state NMR properties of the pyrophosphate moiety for spatially well isolated P2O74- units (such as in Na4P2O7 . 10H(2)O) and for spatially nonisolated P2O74- units (such as in Cd2P2O7) are compared to each other. The properties of these two types of P-31 spin systems under conditions of magic angle spinning (MAS), of two-dimensional spin-echo experiments and rf-driven spin-diffusion experiments on nonspinning samples are investigated. The magnitude of (2)J((PAPB)-P-31-P-31) is determined experimentally for Cd2P2O7.