Continuing their previous studies (This Journal, 58, 300-04 (1950); 59, 18-21, 533-37 (1951); 60, 386-90 (1952); 61, 379-82, 435-40 (1953); 62, 28-33, 618-22 (1954), the authors reported in the present paper the results of their further studies on the reactions on heating hydrated products (calcium aluminate hydrate, xCaO⋅yAl2O3⋅zH2O) of alumina cement. The chief results were summarized as following: (1) Three kinds of samples of alumina cement were taken from three different factories. The ratios of Al2O3 to CaO of these samples were 1.7, 1.3 and 1.1 respectively. Two kinds of calcium aluminate, CaO. Al2O3 and 3CaO⋅5Al2O3, were synthesized as standard samples for the study on the hydraulic binding agents in the hydraulic refractory mortar and concrete. (2) The mechanism of thermal dehydrations of hydration products, xCaO⋅yAl2O3⋅zH2O, in these three samples of alumina cement and two synthesized calcium aluminates above mentioned were studied by the following three methods: (a) differential thermal analysis, (b) thermo-balance and (c) measurement of thermal expansion. (3) Two peaks were observed in dehydration curves on heating the hydrated products; (a) one about 200°C seemed to be due to the hydarted products xCaO⋅yAl2O3⋅zH2O and (b) the other about 300°C to Al2O3 xH2O (or Al(OH)3). (4) This thermal dehydration drew to the end about 730-780°C, and the largest contraction which amounted about 38% was observed in the hydrated product in the sample of the ratio of 1.7. (5) About 900-950°C, secondary product, xCaO⋅yAl2O3, was observed. And then the exothermic reaction was observed at 1000-1010°C with the formation of α-alumina. (6) The dehydrated mass from hydrated alumina cement was not sintered, and they were poor in mechanical strengths. Therefore, some kinds of mineralizer or sintering admixture are necessary to get better strength.