Hypercrosslinked polymers are remarkable materials exhibiting extremely high apparent surface area and exceptional sorption properties. Their special morphology is a result of the fixation of expanded position of polymer chains in a good (swelling) solvent by methylene crosslinking bridges created by the Friedel-Crafts reaction of chloromethyl groups. We have performed a detailed investigation of the formation of the crosslinking bridges in the reaction starting from the swollen, already chloromethylated polymer. Conversion of the chloromethyl groups was determined from the residual chlorine content. The total chlorine content determined after burning a sample of the polymer in oxygen and the content of reactive, accessible chlorine determined by reaction of the chloromethyl groups with pyridine were differentiated. Changes in the content of both types of chlorine were monitored for varying reaction conditions (time, catalyst content, etc.) and were correlated with the development of the microporous texture resulting from the hypercrosslinking of the swollen polymer matrix.