Processes for providing a PEGylated protein composition comprising mono-PEGylated protein and oligo-PEGylated protein, and processes for providing mono-PEGylated protein compositions with high yield and productivity are provided. The processes are particularly suitable for providing mono-PEGylated erythropoietin composition. The processes comprise reacting a non-PEGylated protein with a PEGylation reagent to produce a mixture comprising non-PEGylated, mono-PEGylated and oligo-PEGylated protein, subjecting the mixture to a step of anion exchange chromatography, and recycling non-PEGylated protein into further PEGylation reactions
