Drent and Budzelaar have proposed two catalytic cycles for cationic Pd-diphosphane complex catalyzed CO-alkene coupling reactions such as CO-ethene copolymerization and methoxycarbonylation of ethene. [1] The hydride cycle is initiated by alkene insertion into a Pd-hydride bond whilst the carbomethoxy cycle is initiated by CO insertion into a Pd-methoxy bond. The two cycles may operate in parallel, for example, CO-alkene copolymerization, or one cycle may dominate, for example, the Lucite process for the methoxycarbonylation of ethene to methylpropanoate. Where the two cycles operate in tandem there exists the possibility of crossover between the cycles which has been inferred from chain end-group analysis. [1,2] Product selectivity is determined in both the initiation and termination step(s). [3][4][5] The chainpropagation steps, insertion of CO into a metal-alkyl bond, and insertion of alkene into a metal-acyl, bond have been extensively investigated, [6][7][8][9][10] whilst van Leeuwen has recently discussed termination by alcoholysis of the Pd-alkyl [11] and Pd-acyl intermediates. [4] We have recently (1) fully characterized the intermediates in and (2) delineated the initiation process of the hydride cycle. [12][13][14] The hydride cycle is thus well established, however, a complete cycle for the Pd-carbomethoxy mechanism has not yet been demonstrated because of the absence of suitable Pd-carbomethoxy precursors. [15][16][17][18] We report herein a simple, general procedure for the preparation of the key cationic monocarbomethoxy Pd-diphosphane compounds and present an NMR study that characterizes, for the first time, all the intermediates in the carbomethoxy cycle. We also report (1) the delineation of the propagation steps of the hydride cycle for this catalyst, and (2) for the first time, characterization of all the complexes involved in termination of the hydride cycle with concomitant crossover and initiation of the carbomethoxy cycle. It should be noted that ligands of the type used in this work give highly active catalysts in this field of chemistry. [3]