Structure sensitivity of CO2 hydrogenation reaction catalyzed by Pt/carbon and Pt-M/carbon (M = Ca, Mg) catalysts has been studied. Catalytic activity experiments have been carried out by TPR (temperature programmed reaction). Various techniques like H-2-TPD (hydrogen temperature programmed desorption), TEM (transmission electron microscopy), and XRD (X-ray diffraction) have been used to study the evolution of platinum particles structure. The effects of a heat treatment at high temperature (1200 K) under different atmospheres and the catalyst reduction conditions on the catalytic activity and on the catalyst structure have been analyzed. The results of catalytic activity in relation to particle size have been analyzed with the concept of reaction dimension. The results show that CO2 hydrogenation catalyzed by carbon-supported platinum catalysts is highly sensitive to the structure of the platinum particles. The reaction dimension obtained is close to 0.06. A heat treatment to high temperature (1200 K) produces a large increase in activity. This behavior has been explained considering that at these temperatures, carbon atoms can dissolve in platinum producing a change in the structure, as a result of which the number of active sites increases noticeably.