In this study a 2D transient temperature distribution in a onedimensional thermochemical solar reactor of gas-particles is presented. The gas-particles medium is contained in a reactor where an endothermic chemical reaction of some oxide-metal at high temperature is expected to be occurred. The reactor receives the concentrated solar power on the reactor walls, or directly to the gas-particles medium. As expected the temperature distribution in the reactor is quite homogeneous when it operates to close steady state conditions, but also due to the fact that the inlet cold gas is reheated with the exit warm gas. However, when the reactor starts to operate, the temperature distribution is non-homogeneous depending on how the reactor receives the concentrated solar power.