The experimental electron density of ammonium dihydrogen phosphate (ADP) crystal in the paraelectric phase (155 K) as well as antiferroelectric phase (100 K) is obtained from its high resolution X-ray diffraction data using the maximum entropy method. Marked redistribution of electron density has been observed in ADP crystals as the crystal temperature is lowered below the phase transition temperature Tc=148 K. The nature of very strong O–H–O hydrogen bonds between phosphate anions changes from an ideal covalent interaction to a polar covalent interaction as the temperature is altered from 155 to 100 K. The influence of intermolecular interaction like the dipolar interaction on the electron density particularly in the intermolecular region is clearly visible in the electron density maps. One of the most striking features of the electron density of ADP is the presence of non nuclear maxima (NNM) within the ‘‘ab’’ planes. It is argued that the appearance of these NNMs is a normal consequence of the chemical bonding between homonuclear groups in ADP.