Key indicatorsSingle-crystal X-ray study T = 298 K Mean '(P±O) = 0.002 A Ê R factor = 0.031 wR factor = 0.075 Data-to-parameter ratio = 20.7For details of how these key indicators were automatically derived from the article, see http://journals.iucr.org/e. # 2001 International Union of Crystallography Printed in Great Britain ± all rights reserved Ammonium zinc phosphate, (NH 4 )Zn(HPO 4 )(H 2 PO 4 ), is built up from in®nite 4-ring chains of vertex-sharing ZnO 4 and (H/ H 2 )PO 4 tetrahedra [d av (ZnÐO) = 1.943 (2) A Ê and d av (PÐO) = 1.534 (2) A Ê ] crosslinked by ammonium cations. The intrachain OÐHÁ Á ÁO hydrogen bond appears to be essentially symmetric [d(OÁ Á ÁO) = 2.442 (3) A Ê ].Comment