This invention relates to the use of metal (VI) oxo complexes to catalyse the selective oxidation of 5hmC residues in polynucleotides to 5fC residues. This may be useful in the identification of modified cytosine residues in a population of polynucleotides comprising a sample nucleotide sequence. A first portion of the population is oxidised with a metal (VI) oxo complex and then the first portion and a second portion of said population are both treated with bisulfite. The residues in the first and second portions that correspond to a cytosine residue in the sample nucleotide sequence are identified following treatment and the identities of these residues are used to determine the modification of the cytosine residue in the sample nucleotide sequence. Methods, reagents and kits are provided.