Surface growth is one of the major steps in the soot formation processes where most of the total mass is formed. Freshly generated soot from an ethylene diffusion flame was sampled and mixed with ethylene in a flow tube at elevated temperatures and using nitrogen as the carrier gas. The changes in the size of the soot particles after hydrocarbon addition were measured using a differential mobility analyzer. These measurements showed a significant mass increase of soot particles as a result of the hydrocarbon addition where the total number density remained the same. Soot surface growth kinetics were obtained from the changes in the size, surface area and volume distributions. The activation energy (Ea) for ethylene was obtained. Pure pyrolysis experiments for ethylene were also performed. The surface growth process yield was found to be higher than the yield of the pyrolysis process.