X-ray powder diffraction data are reported for the trihydrated (A3N), the dihydrated (A2N), and the anhydrous ammonium trioxalatoaluminate (III) (AN). A3N is triclinic with space group . A2N and AN are monoclinic with space groups P21/c and P21/n respectively. The stability domains are A3N: –7.8 to 86 °C; A2N: 86 to 102 °C; AN: 102 to 180 °C.