Adsorption and thermal decomposition of trimethylphosphine (PMe3) and triethylphosphine (PEt3) on the Cu(110) surface are investigated using temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS) with synchrotron radiation. Both PMe3 and PEt3 chemisorb molecularly on three adsorption sites on a Cu(110) surface at 100 K and are mostly desorbed intact upon annealing to 520 K. However, a fraction of chemisorbed PMe3 and PEt3 undergoes decomposition via dealkylation. Chemisorbed PEt3 undergoes thermal decomposition to a greater extent than PMe3. PMe3 molecules decompose thermally to form surface CH3 and phosphorus through PMe2 as an intermediate. Surface CH3 eventually disproportionates to form CH4 and surface carbon at a temperature above 420 K. Chemisorbed PEt3 molecules undergo stepwise deethylation (PEt3 --> PEt2 --> PEt --> P), resulting in surface C2H5 and phosphorus: the surface C2H5 moiety further decomposes to evolve C2H4 and H-2 through beta-hydride elimination. Hence a Cu film grown from a CVD precursor containing PEt3 ligands is almost free of carbon, but however, may be deposited with a higher concentration of phosphorus, relative to PMe3.