The present application is directed to a method for performing a bisulfite reaction to determine methylation positions in a nucleic acid, i.e. methylated and non-methylated cytosines. In this method, the nucleic acid is bisulfite treated and is bound to a solid phase when an alkaline solution is added for desulfonation and elution of the nucleic acid from the solid phase. The nucleic acid can be amplified in an additional step. The solid phase is preferably a material comprising glass or silica, more preferably a glass fleece, glass membrane or a magnetic glass particle. Further, several uses of the alkaline solution for bisulfite treatment are disclosed and a kit containing a bisulfite reagent, a solid phase and an alkaline solution.