Intramolecular spin relaxation may occur in isolated molecules or clusters provided that the density of rovibrational eigenstates is sufficiently high to serve as an energy bath and angular momentum is conserved. In the coupled, zero-field limit, total angular momentum (J) is the sum of spin (S) and rotational (N) momenta such that J and M-J are good angular momentum quantum numbers. In the coupled limit, transitions between Zeeman levels (DeltaM(J)not equal0) cannot occur in the absence of an external torque. However, in the high-field limit, J and M-J are no longer good quantum numbers, as N and S are decoupled and only their projections on the z axis defined by the external field are invariant. In this case M-N and M-S remain as good quantum numbers so that angular momentum conserving transitions can occur subject to the selection rule DeltaM(N)=-DeltaM(S). Determination of the magnetic moments of isolated molecules and clusters via a thermodynamics-based analysis requires that their magnetizations are measured at sufficiently large fields that spin-rotation effects become negligible and the Zeeman level structure approaches the free-spin case. (C) 2004 American Institute of Physics.