Polyketones are obtained by the copolymerization of carbon monoxide and ethylene in the presence of a palladium catalyst formed in situ from Pd(OAc)(2), dppp and formic acid in methanol which acted as a solvent. The productivity (g polymer/g Pd* h) is significantly influenced by the concentration of formic acid. Using the ratio Pd/dppp/HCOOH = 1/1/3000 at 90 degreesC and 45 atm, a productivity of 7500 h(-1) is obtained. It is suggested that the efficiency of HCOOH as a promoter is due to its capability to act as a source of Pd-hydride species, which initiates the catalytic cycle. (C) 2002 Elsevier Science B.V. All rights reserved.