Determination of the crystal structure of 'bis (pyridine)silver(I) perchlorate' by single-crystal X-ray methods shows it to be of 2.4:1 pyridine:silver (I) perchlorate stoichiometry , the structure being refined by least squares to a residual of 0.054 for 1142 'observed' reflections. Crystals are tetragonal, I4, a 21.95(1), c 7.684(3) ?. The structure comprises perchlorate anions of two types of I and 4 symmetry, together with two types of complex cation: [Ag( py )2]+, with 1 symmetry, and [Ag( py )4]+ with 4 symmetry. For the two types of cation Ag-N are 2.15(1), 2.16(1) and 2.30(1) ?.