Hydrogen conformations in crystalline H2SO4· 8H2O and H2SO4·6.5H2O have been studied using a system developed by Hirsch [(2003), Z. Anorg. Allg. Chem. 629, 666-672]. New H-atom coordinates, as estimated from DFT calculations, are given for these structures. © 2004 International Union of Crystallography Printed in Great Britain - all rights reserved.