Dissections of solid samples partially reacted under different conditions were examined to determine the control region of the reaction between titanium pellet and nitrogen gas. The thickness of the pellet studied was varied from 0.0012 to 0.00638 m while temperature was varied from 1,173 to 1,473 K. Experimental results indicate that the system is in the chemical reaction control region as the pellet thickness is 0.0012 m for all temperatures studied. The system is in the gas-pore-diffusion control region when the thickness is 0.00638 m and the temperature is 1,373 or 1, 173 K. For other cases, the system is in both control region.