The crystal structures of 3-aminomethyl-1,2,4-benzothiadiazine-1, 1-dioxide (1) and 6-aminomethyl-3-methyl-1,2,4-benzothiadiazine-1,1-dioxide hydrochloride monohydrate (2) have been determined by X-ray crystal data. The benzothiadiazine ring is planar in (1), owing to space-group requirements, and almost planar in (2) with the sulfonyl group perpendicular to the rings. The isomer 4H is peculiar to structure, (2) while (1) contains both the 2H and the 4H isomers. In compound (1) the 4H isomers are connected in pairs through hydrogen bonding [N(4)-H(4)⋯N(3)I 1.88(8) A], while compound (2) is dominated by a network of hydrogen bonding involving mainly the protonated N(3) nitrogen, the crystallized water molecule and the chlorine.