The decomposition of Al-2(1+x)MgxTi(1+x)O-5 solid solutions with x = 0.0, 0.1, 0.2, 0.4, 0.5 and 0.6 was studied in the temperature range 900-1175 degrees C using a 250 h annealing test. As x increases from 0-0.2 there is a strong stabilizing effect and the decomposition temperature decreases from 1280 degrees C (Al2TiO5) down to approximate to 1125 degrees C. For 0.2 less than or equal to x less than or equal to 0.5 the decomposition temperature does not decrease further. For x = 0.6 no decomposition was observed. For x less than or equal to 0.5 decomposition is complete or almost complete at 1000 degrees C; at 900 degrees C transformation is kinetically hindered and solid solutions with x = 0.2 and 0.4 are unaffected by the thermal treatment. A relationship between the decomposition temperature and the parameter x has been derived using the regular solution model to describe the Al-2(1-x)MgxTi(1+x)O-5 solid solution.