The structure of tetragonal NH4H2PO 4 has been determined in a single-crystal neutron-diffraction study. The N-If • • • O and O-H • • • O bond systems suggested in earlier X-ray investigations were confirmed. The H2PO [ network is very similar to that found in KH2PO 4 ; the only significant difference observed was in the angle between the P-O and O-i • • • O bonds. The ammonium ion was found to be only slightly distorted from a regular tetrahedral arrangement. Considerable distortion would be necessary for linear N-i • • • O bonds. The observed angle between the Eli bond and the line joining nitrogen and oxygencenters was about 14 °. In order to achieve good agreement between calculated and observed data it was necessary to introduce anisotropic thermal vibration parameters.The final value of the discrepancy factor was 8.9 %.

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