Molecular motions and chemical exchange processes in glass-forming melts of the phosphorus-selenium system are examined by variable-temperature P-31 and Se-77 NMR spectroscopy. With increasing temperature, the spectra are initially narrowed by motional averaging and subsequently affected by chemical exchange. The Se-77 NMR spectra reflect exchange between Se-bonded and P-bonded Se atoms, which are characterized by a large difference in their respective mobilities. Temperature-dependent measurements of exchange rate constants indicate an activation energy of 67 kT/mol. The P-31 NMR spectra are governed by the interconversion of various types of phosphorus-bearing fragments, which also experience differences in mobility. Detailed simulations are presented for two glasses, containing 13 and 45 atom % phosphorus. Quantitative agreement with the experimental data is achieved using a multisite exchange model with two independent rate constants and a wide distribution of interconversion rates. Simpler models do not reproduce the experimental data satisfactorily. Temperature-dependent simulations of these spectra show that the activation energy for reorientation and chemical exchange Lies in the range 50-60 kT/mol.