Several 1:1 and 2:1 complexes of N-methylmorpholine betaine (MMB, N-(carboxymethyl)– N-methylmorpholinium inner salt) with HCl, HBr, HNO3, HBF4, HI and HClO4 were synthesized and their FTIR, 1H and 13C NMR spectra were investigated. In the 1:1 complexes, MMB·HX, a proton is transferred from the acid to the betaine molecule and both the νOH and νCO frequencies vary with the proton acceptor properties of the anion. The spectra of the 2:1 complexes, (MMB)2H·X, show a broad and intense νOHO stretching absorption in the 1500–400cm−1 range which is slightly affected by the anion and are similar to that for the type A acid salts of carboxylic acids. The crystal structure of bis(N-methylmorpholine betaine) hydrochloride [(MMB)2H·Cl] was solved by X-ray diffraction as monoclinic, space group C2/c, a=23.113(2)Å, b=6.9404(6)Å, c=10.800(1)Å, β=94.450(7)°, Z=4. The carboxylate groups of a pair of MMB are bridged by a hydrogen atom to form a dimeric cation [(MMB)2H]+ with a very strong, linear and symmetrical O⋯H⋯O hydrogen bond of the length 2.435(2)Å. The centers of cations and anions are located in special positions, in the inversion centers and on twofold axes, respectively.