The nature of the co-ligand X drives the primary loss in the MS processes of [Re(CO) (PO)(X) + H] compounds. When X = solvent, the energetics of these decompositions follow the trend H O < MeOH < CO. In each case, loss of CO is a favored fragmentation route with associated energies following the trend: N-py ≤ P-phosphine < C-isonitrile. Overall, MS pathways indicate that [Re(PO)] (Re with chelated PO phosphine) constitutes the residual moiety. This behavior indicates that the presence of a functionalized phosphine is essential for a sequential, controlled release of CO.