From on-line coupled TGA-MS and TGA-FTIR measurements, in combination with a quantitative chemical analysis, it was deduced that the chemical formula for an unknown bismuth oxalate compound had to be Bi(NH4)(C2O4)(2).3.71(6)H2O. Solution of the crystallographic structure on the basis of X-ray powder data proved this formula to be correct. The diffraction pattern was indexed by a tetragonal unit cell [a and c respectively 11.6896(2) and 9.2357(3) Angstrom; M-20 = 195 and F-30 = 302; Z(calc) = 4], from which the space group /4(1)/amd (No. 141) was derived. Direct methods were applied to solve the structure. The initial structural model was subsequently refined by means of the Rietveld method (R-B = 8.0%, R-wP = 14.0%). Bi is 8-fold coordinated by oxygen from the oxalate anions. Since these BiO8 polyhedrons do not share any edges or vertexes, an open framework is formed with water and ammonium molecules between. As a result, water can easily be removed, which is clearly indicated by the instant weight loss in the TGA upon heating. Moreover, as shown by HT-XRD, this process of water exchange is reversible as long as the heating temperature does not exceed 100 degreesC.