Abstract In this work the steady and unsteady flamelet models have been applied to soot formation in laminar and turbulent diffusion flames. The aim was to study how different model parameters affect soot formation in diffusion flames. It was shown that certain assumptions are more crucial in laminar diffusion flames than in turbulent ones. The soot formation in turbulent diffusion flames is more sensitive to the surface dependence of the particle and altering the active site parameter, than in laminar flames. This is due to the fact that the flame is laminar and the turbulent mixing, which supplies the particle with radicals, does not affect the process. The active site parameter decreases in the absence of radicals [1]. The modeling of complex diffusivity of all species is more relevant in laminar diffusion flames than in turbulent diffusion flames. All transient effects investigated in this work were shown to affect soot formation, which is itself transient. It was shown that these effects are more relevant for laminar flames than for turbulent flames. The steady flamelet model allows the inclusion of many transient processes and thereby loses in accuracy compared with the unsteady model. Finally, the process of the formation of agglomerates was included in the unsteady model and it was shown that the soot volume fraction is affected when this process is considered.