Electron impact (EI) mass spectrometer apparatus has been used to monitor the relative intensities of ion clusters of the type Xn(H2O)n where X is 3-arylpyrimido[4,5-c]pyridazine-5,7(6H,8H)-diones (1a-d), 3-aryl-7-thioxo-7,8-dihydro-6H-pyrimido[4,5-c]pyridazine-5-ones (2a-d) and n = 1, 2. The m/z of selected fragments obtained from 1a-d and 2a-d show a clustered water molecule due to strong intermolecular H-bonding between fragment and clustered water molecule.