The crystal structure of imidazole imidazolium perchlorate has been determined at room temperature from three-dimensional diffractometer data, and refined by full-matrix least-squares to a final R = 0.051. The planar imidazole rings are linked by a [Formula: see text] hydrogen bond of length 2.73 Å and rotated with respect to each other about this bond by an angle of 50°. The analysis has failed to give conclusive evidence as to the position of the hydrogen atom between the two imidazoles.