The structure of acid strontium oxalate Sr(HC2O4) . 1/2(C2O4) . H2O has been determined by conventional X-ray powder diffractometry, The diffraction pattern was indexed from a monoclinic unit cell with cell parameters a = 6.341(1) Angstrom, b = 16.880(2) Angstrom, c = 5.7798(8) Angstrom, and beta = 97.60(1)degrees; space group, P2(1)/n (No. 14) with Z = 4. Final (isotropic) Rietveld refinement of the neutron powder data yielded R-B = 4.57%, R-F = 2.87%, and R-WP = 7.87% as conventional Rietveld parameters. Strontium is eight-fold coordinated and can be described as a distorted bicapped trigonal prism. The SrO8 polyhedra form one-dimensional chains along the c-axis bf sharing edges, In contrast to all other known strontium oxalates, in this compound H2O acts as bridging ligand between two Sr atoms, SDPD-D classification (1): Sr(HC2O4)-1/2(C2O4)-H2O, P2(1)/n, C1=14, Nc = 42, C2 = 11, XC1 + N/TREOR-97 & DICVOL-91, EXTRA, EQUI, SIRPOW-92 (DM), GFOURIER, FULLPROF, (C) 2001 Academic Press.