Source of materialCaffeine and perchloric acid in a molar ratio of 1:1 were mixed and dissolved in sufficient amount of water by heating to a temperature where a clear solution resulted. Crystals of the title compound were formed by gradual evaporation of water over a period of one week at 293 K. Experimental detailsPerchlorate anion (including Cl atoms) is disordered and described by two positions with occupancies of 0.5. ClO and O···O distances were restrained to 1.42(3) Å and 2.34(3) Å, respectively. H atom attached to N atom was deduced from Fourier difference map and refined freely. H atoms of water molecule could not be localized. Other H atoms were placed in calculated positions and allowed to ride on their parent atoms at distances of 0.93 Å (C5H5) and 0.96 Å (methyl) with U iso (H) values 1.2 times U eq of the parent atoms. DiscussionCaffeine (3,7-dihydro-1,3,7-trimethyl-1,4-purine-2,6-di-one) is a purine alkaloid possessing pharmacological properties as a therapeutic agent with analeptic activity.