Single-crystal diffraction data have been recorded for a very small crystal of dimensions 8 x 18× 175~ rn3. From these data the structure of piperazine silicate (EU 19) has been determined and refined to R= 0.094 for 490 observed reflections. Data collection for such a crystal has been made practicable by the high intensity of the Daresbury Synchrotron Radiation Source and the Enraf-Nonius FAST area detector diffractometer. The potential of this synchrotron radiation method for other small crystals including proteins is discussed. Crystal data: 2+• 2-C4H~ 2N2. 816013, Mr= 464-6, monoclinic, C2/c, a= 13.57 (2), b= 4.90 (1), c= 22.46 (3) A, fl= 91.67 (1), V= 1493A3, Z= 4, Dx= 2.04 gcm-3, 2= 0.90 A, F (000)= 952, room temperature. The structure contains silicate double layers separated by, and hydrogen bonded to, piperazinium ions.