A process for producing PEEK-PEDEK copolymer consisting essentially of repeat units of formula I: -O-Ph-O-Ph-CO-Ph- I and repeat units of formula II: -O-Ph-Ph-O-Ph-CO-Ph- II, and end units, the process comprising a nucleophilic polycondensation of a mixture of dihydroxybenzene and dihydroxybiphenyl in a molar ratio of dihydroxybenzene:dihydroxybiphenyl from 50:50 to 95:5, with 4,4'-dihalobenzophenone in a reaction mixture comprising sodium carbonate and potassium carbonate in an aromatic sulfone solvent, at a reaction temperature rising to a temperature from 280°C to 330°C immediately prior to; b addition of a salt, which is a lithium, magnesium, calcium barium or strontium salt, or a mixture thereof, to the reaction mixture, wherein the molar ratio of the salt to potassium carbonate is from 6.0 to 10.0; c addition of a further organic dihalide to the reaction mixture, simultaneously with or subsequent to step b, wherein the molar ratio of the further organic dihalide to the mixture of dihydroxybenzene and dihydroxybiphenyl is from 0.009 to 0.035; d maintenance of the resulting reaction mixture's temperature at from 280°C to 330°C for from 20 to 180 minutes; e cooling of the resulting reaction mixture and recovery of the PEEK-PEDEK copolymer resulting from steps a to d from the reaction mixture; wherein in step a of the process: i the molar ratio of sodium carbonate to the mixture of dihydroxybenzene and dihydroxybiphenyl is from 0.95 to 1.15; ii the molar ratio of potassium carbonate to sodium carbonate is from 0.0025 to 0.0040; iii the molar ratio of the 4,4'-dihalobenzophenone to the mixture of dihydroxybenzene and dihydroxybiphenyl is from 1.005 to 1.010; and iv the aromatic sulfone solvent is present in the reaction mixture as 0.30 to 0.63 kg/mole of the mixture of dihydroxybenzene and dihydroxybiphenyl. a nucleophilic polycondensation of a mixture of dihydroxybenzene and dihydroxybiphenyl in a molar ratio of dihydroxybenzene:dihydroxybiphenyl from 50:50 to 95:5, with 4,4'-dihalobenzophenone in a reaction mixture comprising sodium carbonate and potassium carbonate in an aromatic sulfone solvent, at a reaction temperature rising to a temperature from 280°C to 330°C immediately prior to; b addition of a salt, which is a lithium, magnesium, calcium barium or strontium salt, or a mixture thereof, to the reaction mixture, wherein the molar ratio of the salt to potassium carbonate is from 6.0 to 10.0; c addition of a further organic dihalide to the reaction mixture, simultaneously with or subsequent to step b, wherein the molar ratio of the further organic dihalide to the mixture of dihydroxybenzene and dihydroxybiphenyl is from 0.009 to 0.035; d maintenance of the resulting reaction mixture's temperature at from 280°C to 330°C for from 20 to 180 minutes; e cooling of the resulting reaction mixture and recovery of the PEEK-PEDEK copolymer resulting from steps a to d from the reaction mixture; i the molar ratio of sodium carbonate to the mixture of dihydroxybenzene and dihydroxybiphenyl is from 0.95 to 1.15; ii the molar ratio of potassium carbonate to sodium carbonate is from 0.0025 to 0.0040; iii the molar ratio of the 4,4'-dihalobenzophenone to the mixture of dihydroxybenzene and dihydroxybiphenyl is from 1.005 to 1.010; and iv the aromatic sulfone solvent is present in the reaction mixture as 0.30 to 0.63 kg/mole of the mixture of dihydroxybenzene and dihydroxybiphenyl.