The electrospray ionisation mass spectra (EDESI-MS) of Ru6C(CO)16(PPh3) and Ir4(CO)11(PR3) (PR3 = PPh3, P(p-C6H4OMe)3, P(p-C6H4NMe2)3, P(p-C6H4Cl)3, P(OPh)3, P(OMe)3, PO3C5H9) are described and the relative importance of carbonyl loss vs. phosphine loss as a fragmentation pathway is assessed. Qual., the phosphine ligands bind more strongly to Ir4(CO)11 clusters than to Ru6C(CO)16. The influence on the collision cell pressure on MS/MS spectra of transition metal carbonyl cluster anions is also explored showing that a greater, simultaneous, distribution of fragment ions is produced as the collision cell pressure is increased.