The influence of CO and C2H4 pressures on their copolymerization catalyzed by the precursor [(dppp)Pd(H2O)(TsO)](TsO) in MeOH has been studied up to 8 Mpa of total pressure at 363 K. Several rate equations, based on two different models, are presented. The so called "one site model" involves a tetracoordinate Pd-II cation in which only one coordination site is available to the monomers. The "two sites model" involves a pentacoordinate Pd-II cation in which the monomers coordinate the metal simultaneously at two different sites. In both cases, the other three sites are occupied by the growing polymer chain and the chelating diphosphine. The rate equation that fits the experimental data better is used to optimize the ratio of the monomers pressures to obtain a maximum reaction rate for a given total pressure.