A method of conjugating a water soluble polymer to an oxidized carbohydrate moiety of a therapeutic protein comprising the following steps: i) preparing an activated water soluble polymer containing an active aminooxy group by a method comprising: a) incubating a solution comprising an oxidized water-soluble polymer with an aminooxy linker comprising an active aminooxy group under conditions that allow the formation of a stable oxime linkage between the oxidized water-soluble polymer and the activated aminooxy linker, said conditions comprising a time period between 1 minute and 24 hours; a temperature between 2°C and 8°C; in the presence or absence of light, and with or without stirring; thereby forming a water-soluble polymer containing an active aminooxy group; and b) purifying the water soluble polymer containing an active aminooxy group by a method selected from the group consisting of chromatography, filtration, dialysis, and precipitation, at a temperature between 2°C and 8°C; ii) oxidizing a carbohydrate moiety on a therapeutic protein by incubation with a buffer comprising an oxidizing agent selected from the group consisting of sodium periodate (NaIO 4), lead tetraacetate (Pb(OAc) 4) and potassium perruthenate (KRuO 4); iii) contacting the oxidized carbohydrate moiety with the activated water soluble polymer under conditions that allow conjugation; wherein said activated water soluble polymer containing an active aminooxy group is selected from the group consisting of polysialic acid, starch, hydroxyalkyl starch, hydroxylethyl starch, pullulan, chitosan, hyaluronic acid, chondroitin sulfate, dermatan sulfate, dextran, carboxymethyl-dextran, and other polysaccharides containing reducing end groups; and iv) catalyzing an oxime linkage by a nucleophilic catalyst selected from the group consisting of o-amino benzoic acid, m-amino benzoic acid, p-amino benzoic acid, sulfanilic acid, o-aminobenzamide, o-toluidine, m-toluidine, p-toluidine, o-anisidine, m-anisidine, and p-anisidine, wherein said oxime linkage is formed between the oxidized carbohydrate moiety and the active aminooxy group on the water soluble polymer. i) preparing an activated water soluble polymer containing an active aminooxy group by a method comprising: a) incubating a solution comprising an oxidized water-soluble polymer with an aminooxy linker comprising an active aminooxy group under conditions that allow the formation of a stable oxime linkage between the oxidized water-soluble polymer and the activated aminooxy linker, said conditions comprising a time period between 1 minute and 24 hours; a temperature between 2°C and 8°C; in the presence or absence of light, and with or without stirring; thereby forming a water-soluble polymer containing an active aminooxy group; and b) purifying the water soluble polymer containing an active aminooxy group by a method selected from the group consisting of chromatography, filtration, dialysis, and precipitation, at a temperature between 2°C and 8°C; a) incubating a solution comprising an oxidized water-soluble polymer with an aminooxy linker comprising an active aminooxy group under conditions that allow the formation of a stable oxime linkage between the oxidized water-soluble polymer and the activated aminooxy linker, said conditions comprising a time period between 1 minute and 24 hours; a temperature between 2°C and 8°C; in the presence or absence of light, and with or without stirring; thereby forming a water-soluble polymer containing an active aminooxy group; and b) purifying the water soluble polymer containing an active aminooxy group by a method selected from the group consisting of chromatography, filtration, dialysis, and precipitation, at a temperature between 2°C and 8°C; ii) oxidizing a carbohydrate moiety on a therapeutic protein by incubation with a buffer comprising an oxidizing agent selected from the group consisting of sodium periodate (NaIO 4), lead tetraacetate (Pb(OAc) 4) and potassium perruthenate (KRuO 4); iii) contacting the oxidized carbohydrate moiety with the activated water soluble polymer under conditions that allow conjugation; wherein said activated water soluble polymer containing an active aminooxy group is selected from the group consisting of polysialic acid, starch, hydroxyalkyl starch, hydroxylethyl starch, pullulan, chitosan, hyaluronic acid, chondroitin sulfate, dermatan sulfate, dextran, carboxymethyl-dextran, and other polysaccharides containing reducing end groups; and iv) catalyzing an oxime linkage by a nucleophilic catalyst selected from the group consisting of o-amino benzoic acid, m-amino benzoic acid, p-amino benzoic acid, sulfanilic acid, o-aminobenzamide, o-toluidine, m-toluidine, p-toluidine, o-anisidine, m-anisidine, and p-anisidine, wherein said oxime linkage is formed between the oxidized carbohydrate moiety and the active aminooxy group on the water soluble polymer.