Tomographic energy dispersive diffraction imaging (TEDDI) is a recently developed synchrotron-based characterization technique used to obtain spatially resolved X-ray diffraction and fluorescence information in a noninvasive manner. With the use of a synchrotron beam, three-dimensional (3D) information can be conveniently obtained on the elemental composition and related crystalline phases of the interior of a material. In this work, we show for the first time its application to characterize the structure of a heterogeneous catalyst body in situ during thermal treatment. Ni/gamma-Al2O3 hydrogenation catalyst bodies have been chosen as the system of study. As a first example, the heat treatment in N-2 of a [Ni(en)(3)](NO3)(2)/gamma-Al2O3 catalyst body has been studied. In this case, the crystalline [Ni(en)(3)](NO3)(2) precursor was detected in an egg-shell distribution, and its decomposition to form metallic Ni crystallites of around 5 nm was imaged. In the second example, the heat treatment in N-2 of a [Ni(en)(H2O)(4)]Cl-2/gamma-Al2O3 catalyst body was followed. The initial [Ni(en)(H2O)(4)]Cl-2 precursor was uniformly distributed within the catalyst body as an amorphous material and was decomposed to form metallic Ni crystallites of around 30 nm with a uniform distribution. TEDDI also revealed that the decomposition of [Ni(en)(H2O)(4)]Cl-2 takes place via two intermediate crystalline structures. The first one, which appears at around 180 degrees C, is related to the restructuring of the Ni precursor on the alumina surface; the second one, assigned to the formation of a limited amount of Ni3C, is observed at 290 degrees C.