The current work presents a comprehensive study that aims at understanding the role of silicon on θ precipitation, as well as on the ε → θ carbide transition in tempered martensite. Cementite nucleation was modelled under paraequilibrium conditions in order to ensure the presence of silicon in the carbide, where both thermodynamic and misfit strain energies were calculated to evaluate the overall free energy change. The growth stage was investigated using in situ synchrotron radiation; three alloys containing 1.4-2.3 wt.% silicon contents have been studied. Silicon appears to play a significant role in carbide growth. It was observed throughout tempering that cementite precipitation was slower in the higher silicon content alloy. Literature reports that cementite growth is accompanied by silicon partitioning, where the silicon content inside the carbide decreases as tempering progresses. Therefore it appears that the limiting factor of the growth kinetics is the rate at which silicon is rejected from the carbide; the silicon piles up at the carbide-matrix interface, acting as a barrier for further growth.