The invention provides methods for treating methylated nucleic acids. In one embodiment, the method can include the steps of (a) providing an alkali environment to a nucleic acid sample; (b) reacting the nucleic acid sample with a bisulphite reagent and incubating the reaction so as to form a treated nucleic acid sample where methylated nucleotides in the nucleic acid sample remain unchanged while unmethylated nucleotides are converted to another form; (c) diluting the treated nucleic acid sample so as to reduce salt concentration to a level which will not substantially interfere with a nucleic acid precipitating step; (d) precipitating the diluted treated nucleic acid to substantially remove any unwanted reagents or diluents from treated nucleic acid; and (e) carrying out de-sulphonation of the precipitated treated nucleic acid so as to remove sulphonate groups present on the treated nucleic acid so as to obtain a nucleic acid sample substantially free of sulphonate groups.