The dehydration of uranyl nitrate trihydrate is investigated by Controlled transformation Rate Thermal Analysis (CRTA) allowing both the control of the rate and residual pressure. The "abnormal" influence of the residual pressure above the sample, known as the Smith-Topley effect, is highlighted in a lower than usual pressure region, between 10(-1) mbar and 5 x 10(-3) mbar. Two mechanisms are distinguished : in the upper pressure region a mechanism of instantaneous nucleation and 2-dimensional nuclei growth occurs;lowering the pressure favours a 2-dimensional interfacial advancement mechanism. This type of Smith-Topley effect and its interpretation have not been previously proposed.