The nuclear spin conversion is induced by hyperfine intramolecular magnetic spin-spin and spin-rotation interactions. The measurement of the conversion decay rates leads to the determination of these interactions strengths whose physical meaning is analysed in view of new ab initio calculations. It is also shown that the results derived from nuclear spin conversion are complementary to those obtained by conventional spectroscopy. (C) 2004 Elsevier B.V. All rights reserved.