The molecular properties of aminomethylphosphonic acid in the zwitterionic and molecular forms were calculated using the AMI and ab initio methods with the STO-3G∗, 3-21G∗ and 6-31G∗ basis sets at the self-consistent-field and electron-correlation level. It was found that the molecular form of this acid is 10.4 kcal mol−1 more stable than the zwitterionic one in the gas phase.