Abstract: The invention relates to a procedure for chemically determining the base sequence of DNA and, in particular, its step of cleaving the labeled DNA having undergone a base-selective chemical modification at the position of the modified base. More particularly, it relates to a method for cleaving DNA which comprises modifying DNA base-selectively with chemical reagents, bringing the resulting reaction mixture into contact with an adsorbent to adsorb the chemically modified DNA on the adsorbent, washing the adsorbent with a washing fluid to remove any chemical modification reagents from the adsorbent, bringing the adsorbent into contact with an eluent to elute the chemically modified DNA adsorbed thereon, concentrating the resulting eluate and then heating it together with a cleavage reagent.