A new optical diagnostic technique has been used to measure the spatially distributed temperatures, soot diameters, and soot volume fractions in several different ethylene laminar diffusion flames to investigate the effect of adding hydrogen and helium on the soot formation. The test results show that adding hydrogen increases the flame temperature in all regions, while adding helium does not significantly affect the flame temperature in the reaction region but does increase the flame temperature elsewhere. The flame heights when adding helium and hydrogen can be calculated using the correlation introduced by Roper if the ethylene diffusion coefficient is used. This indicates that the flame height is determined by the diffusion of ethylene molecules when the hydrogen fraction is below 20%. It was also found that either adding helium or hydrogen does not significantly affect the soot diameter but does reduce the soot volume fraction. A total of 20% of helium addition by volume was measured to reduce the total soot number by 19%, while a total of 20% of hydrogen addition reduced the total soot number by 23%. In comparison, replacing the hydrocarbon with hydrogen is much more effective in reducing soot formation. Replacement of 25% ethylene by hydrogen was measured to reduce the total soot number by 66%. Apart from demonstrating the influence of hydrogen and helium on ethylene diffusion flames, these measurements provide additional data for modelers of diffusion flames, especially those with an interest in the formation of particulate matter.