A study of the solid-liquid equilibria in the ternary system H2O-NH4NO3-Al(NO3)(3) has been undertaken in order to find an invariant transformation at low temperature. Five isothermal sections have been established under atmospheric pressure at -25, -20, -18.2, -10 and 0 degrees C. The solid phases observed at these temperatures are : at -25 and -20 degrees C, ice, NH4NO3(V) and Al(NO3)(3) . 9H(2)O; at -18.2 degrees C, ice, NH4NO3(IV) and Al(NO3)(3) . 9H(2)O; at -10 degrees C, ice, NH4NO3(IV), Al(NO3)(3) . 9H(2)O and Al(NO3)(3) . 8H(2)O; at 0 degrees C, NH4NO3(IV), Al(NO3)(3) . 9H(2)O and Al(NO3)(3) . 8H(2)O. The X-ray diffraction pattern of the octahydrated aluminium nitrate has been established. The spatial diagram shows four isobaric invariant transformations whose temperatures have been specified by thermal analysis. They correspond to the reactions : At -33.6 degrees C, liquid reversible arrow ice + NH4NO3(V) + Al(NO3)(3) . 9H(2)O At -18.6 degrees C, liquid + NH4NO3(IV) reversible arrow ice + NH4NO3(V) liquid + NH4NO3(IV) reversible arrow NH4NO3(V) + Al(NO3)(3) . 9H(2)O At -12.3 degrees C, liquid + Al(NO3)(3) . 8H(2)O reversible arrow NH4NO3(IV) + Al(NO3)(3) . 9H(2)O