By varying the P-ligand-nickel ratio in the title systems over a three-orders of magnitude range, the selectivity of the decompn. and of the CO-insertion (ligand concn. control) was analyzed and compared with the range of stepwise ligand assocns., identified by 31P NMR spectroscopy. Investigations of the range of existence of the complexes, the degree of CO-absorption and the selectivity of the products formed by means of ligand concn. control maps lead to a subsystem scheme for this system, describing the relations of the various species involved. A method for quant. sepg. the ligand concn. control (derivable from the range of existence of the species) from the property specific ligand control (chem. behavior of the species) was given.