Previous studies have demonstrated that methyl ester-based biodiesels produce significantly less soot compared to petroleum diesel in standard diffusion flames. However, the roles of the oxygen content and the degrees of unsaturation (DoU) of the biodiesels on their sooting propensity have not been independently analysed. In the present study, the relationship between soot yield of biodiesels and the DoU of the fuel is systematically analysed in laminar pool flames and pre-vaporised diffusion flames. The spatial distribution of soot volume fraction of the flames fuelled with four biodiesel fuels, two methyl esters and their blends with diesel are quantitatively measured using laser induced incandescence. All six biofuels have different DoU but similar oxygen mass fractions, so that the effect of the DoU on soot formation could be separately considered. The soot yield using biodiesels was measured to be lower than that of diesel fuel by a factor up to 7.5 in pool flames and by a factor of 4.0 in vapour flames. In both flame setups, the soot reduction effect of biodiesels ranks in order of DoU. The more saturated the fuel molecule is, the lower the soot yield. The soot reduction effect of the blending ratio with biodiesel was found to be non-linear, and different in pool and vapour flames. The morphology, size, and number density of soot particles were analysed using scanning electronic microscopy. The results suggest that the shape of primary soot particles is close to spherical and the structure of the point-contact aggregates is similar in all cases. Biofuels not only produce smaller particle sizes, but also fewer particles compared to diesel. Chemical kinetic simulations suggest that blending of biodiesels reduces the soot yield primarily by slowing the soot surface growth process, and secondarily by prohibiting the soot nucleation and inception. Both the nucleation (C3H3 and benzene) and growth species (C2H2) for soot formation take place more quickly and intensely for the combustion of more unsaturated fuels, which results in the correspondingly larger soot yield.