The structures of the anhydrous morpholinium salts of the isomeric 2-aminobenzoic acid (anthranilic acid) (1), 3-aminobenzoic acid (2) and 4-aminobenzoic acid (3), with formula C4H10NO+ C7H8NO2 −, have been determined. Crystals of 1 are monoclinic, space group P21 /c, with unit cell dimensions a = 11.0696(9), b = 7.8775(6), c = 13.0180(11) A, β = 95.804(8)° and Z = 4. Compound 2 is orthorhombic, space group Pna21, with unit cell dimensions a = 9.3899(4), b = 16.6789(7), c = 7.4463(3) A and Z = 4 and compound 3 is monoclinic, space group Cc, with unit cell dimensions a = 5.9233(6), b = 18.048(2), c = 10.5478(10) A, β = 90.376(10)° and Z = 4. In all crystals, both morpholinium N–H atoms are involved in primary inter-species hydrogen-bonding interactions with carboxyl O-atom acceptors of the anions. Secondary N–H···O hydrogen bonds between the amine groups and carboxyl (1 and 3) or both carboxyl and morpholine O-atom acceptors in 2, including in 2 a three-centre asymmetric N–H···O,O’ chelate [ $$ R_{1}^{2} $$ (4)] motif, give two-dimensional layers (1 and 2) or a three-dimensional network structure (3). In 1, the interactions include a centrosymmetric cyclic $$ R_{4}^{4} $$ (12) motif and in 2, a cyclic $$ R_{3}^{2} $$ (11) motif but no π–π aromatic ring associations are present in any of the structures. The structures of the anhydrous morpholinium salts of the three isomeric monoaminobenzoic acids are reported, together with their hydrogen-bonding patterns.