Hydroesterification of ethylene to methyl propionate has been studied using the catalyst precursor Pd(PPh3)(2)(TsO)(2), which is active in presence of PPh3 and TsOH (TOF = 5700 h(-1) at 120 degreesC, 40atm (CO/C2H4 = 1/1), Pd/PPh3/TsOH = 1/8/10, [Pd] = 2 x 10(-3) mol1(-1), solvent methanol, H2O = 800 ppm). In this paper we study the promoting effect of a hydride source, molecular hydrogen, water and p-toluenesulfonic acid (TsOH) and the inhibiting effect of p-benzoquinone. On the basis of experimental evidences, of the two possible initial steps of the catalytic cycle, the insertion of the olefin into a Pd-H species or into a Pd-OCH3 species, it is suggested that the first plays a more important role.