1-Carboxymethyl-1'-carboxylatomethyl-3,3'-[p-phenylenebis(oxymethylene)]dipyridinium bromide dihydrate.

In the crystal structure of the title salt, C(22)H(21)N(2)O(6) (+)·Br(-)·2H(2)O, pairs of betaine mol-ecules are bridged by protons (the bridging proton is disordered), forming strong and symmetrical O-H⋯O hydrogen bonds, leading to an infinite chain along the b axis. The water mol-ecules are linked to the betaine mol-ecule and the bromide ion through O-H⋯O and O-H⋯Br inter-actions. The central ring, located on an inversion centre, makes dihedral angles of 1.2 (2)° with the outer rings. One of the carboxylic acid groups is deprotonated.