A process is described for preparing, efficiently and with a high degree of purity N-alkylsulfonates of phenothiazine. The process consists in (a) the preparation of the phenothiazine anion, and (b) the reaction of said anion with cyclic alkyl sulfonates, such as 1,3-propane sultone and 1,4-butane sultone. This process is simpler, more direct, and more efficient than the procedures currently used for the synthesis of N-alkylsulfonates derivatives of phenothiazine. In addition, the products obtained with this process are far more pure than those obtained through current procedures and therefore ideal for bioanalytical applications that require high sensitivity, such as assays based on the measurement of peroxidase activity using chemiluminescence.