Gamma uranium-molybdenum alloys have been considered as the fuel phase in plate type fuel elements for material test reactor (MTR) reactors, mainly owing to their acceptable performance under irradiation and metallurgical processing. Regarding their usage as a dispersion phase in aluminum matrix, it is necessary to convert the as cast structure into powder, and one of the techniques considered for this purpose at IPEN/CNEN (Brazil) is by means of the HDH (hydration-dehydration) reactions. This paper shows that, under specific conditions of heating and cooling, hydration-dehydration is sometimes unnecessary, and gamma UMo fragmentation occurs in a nonreaction predominant mechanism, as shown by the curves of hydrogen absorption/desorption by the gamma uranium-molybdenum alloys as a function of time and temperature. Our focus was on the experimental results presented by the addition of 8% weight molybdenum. Subsequent to their production by induction melting, samples of the alloys were thermally treated under a constant flow of hydrogen for temperatures varying from 500 to 600 degrees C and for time of 2 to 4 h. The curves relating mass incorporation and time for the above-mentioned temperatures were obtained, and the results were related to the fragmentation. The main conclusion is that, even without a massive hydration dehydration process, the samples could be fragmented under specific conditions of thermal treatment under the thermal shock phase of the experiments and that there is a strong correlation between absorption and the rate of gamma decomposition or the gamma phase stability of the alloy. In addition, a suggested value of 568.4 degrees C was obtained for the maximum rate of gamma decomposition, which can also be related to its eutectoid transformation temperature.