One of the most intriguing facts about hydrogen bonds is that bonds formed between the same couple of donorand acceptor atoms can have large variation in the geometry as well as energies; this is mainly because hydrogen bonds arehighly sensitive to their environment.We report here a comparative study of neutron structures of mixed crystals of potassium–ammonium dihydrogen phosphate. Basic structural framework of crystals belonging to potassium dihydrogen phosphatefamily is constructed by a network of zigzag O–H–O hydrogen bonded chains of PO$_2$(OH)$_2$ anions, with cations occupying the interspaces of the zigzagged anionic chains. These crystals having simple crystal structure primarily determined by theO–H–O hydrogen bonds connecting PO$_2$(OH)$_2$ anions provide us with a unique opportunity to tune the hydrogen bondgeometry by varying the cationic occupancy and study the effect of this change on the average structure of the crystal. Thecation–anion interaction is found to have a direct influence on the O–H–O hydrogen bonds of the mixed crystals.