The line-of-sight soot surface temperatures and soot volume fractions were measured as a function of axial position in overventilated coflow laminar diffusion flames. A comparison of the influence of nitrogen dilution and the flame temperature on soot formation in diffusion flames of ethylene was made, and the relative importance of the two effects was quantified. To isolate the influence of the dilution and flame temperature, a reference condition was specified, such that the temperature of the reactants was 623 K without nitrogen dilution of the fuel gas. In dilution experiments, the temperature of reactants was at the reference temperature and the nitrogen dilution of the ethylene was varied from 0 to 0.78 mole fraction. In a second set of experiments, where dilution was zero, the flame temperature was varied by reducing the temperature of the reactants from 623 to 298 K in six steps. The reduction in soot formation is due to both lowered temperature and fuel concentration in dilution experiments, and due to only lowered temperature in undiluted flames since the temperature was varied by controlling the temperature of the reactants. The difference between the two sets of results, for the same flame temperature, gives the influence of dilution on soot formation. The observed soot concentrations in both diluted and undiluted flames, after allowance is made for differences in flame heights and diameters, are fitted to a simple Arrhenius rate expression where the preexponential factor is linear in fuel mole fraction and the measured activation energy is 200 Ll/mol. This rate expression correctly predicts the observation that the effect of dilution on the maximum soot volume fraction is greater than that of temperature up to a diluent fraction of about 0.7. At this point the contributions of both factors are equal; further increase in the diluent amount makes the contribution of the temperature larger.