The neutral precursor cis-[Pd(SO4)(PPh3)(2)] turns into an active catalyst for the hydromethoxycarbonylation of ethene when used in combination with H2SO4 and PPh3. The influence of the following operating conditions on the catalytic activity have been studied: (i) H2SO4/Pd ratio; (ii) PPh3/Pd ratio; (iii) total pressure with CO/ethene = 1/1; (iv) pressure of one gas at constant pressure of the other; (v) H2O concentration; (vi) temperature. At 100 degrees C a TOF = 2168 h(-1) has been achieved when the catalytic system is used in the ratios Pd/H2SO4/P = 1/107/18 (mol/mol), under 6 bar (CO/E = 1/1), H2O concentration 0.16% in MeOH by weight. After catalysis and upon addition of LiCl, trans-[Pd(COEt)Cl(PPh3)(2)], which is related to the "Pd-H" catalytic cycle, has been isolated. Cis-[Pd(SO4)(PPh3)(2)] in CD2Cl2/MeOH reacts with CO to give a PdCOOMe complex (related to the "carbomethoxy mechanism"), which neither inserts ethene, nor gives methyl propanoate (MP). In the presence of H2O and H2SO4 the carbomethoxy complex is unstable giving a Pd-H complex, which yields catalysis to MP in the presence of CO and ethene. The "Pd-H" and "Pd-COOMe" catalytic cycles are discussed on the basis of the influence of the operating conditions on the TOF and of NMR evidences. (C) 2010 Elsevier B.V. All rights reserved.