The present work is devoted to research the influence of micro-structural evolution of the compound zone (iron (carbo)nitrides zone) upon development of hardness profiles in diffusion zone. A different phase structure of iron (carbo)nitrides zone on steel as compared to iron, further changing with the process, may result in upsetting the quasi-equilibrium of nitrogen concentration in the iron (carbo)nitrides zone/diffusion zone interface and may as a result have impact on the kinetics of this layer’s growth. Aimed at solving this problem there was a research carried out to evaluate influence of (carbo)nitrides zone, with intentionally created diametrically different phase composition, on hardness profiles in the diffusion zone. Based on the research conducted, it was shown that the evolution of phase structure of the compound zone contributes significantly, regardless of nitrogen potential and temperature, to the formation of the diffusion zone and in particular to its effective thickness. It makes this complex picture of nitrided case development on steel even more intricated.