The smoke points of laminar ethylene diffusion flames have been measured under various conditions of dilution. Nitrogen, argon, helium and carbon dioxide have been introduced as diluents to both the oxygen and fuel streams of the burners. In the former case measurements have been made wherever possible over the oxygen index range 0.12 to 0.45 (12% to 45% oxygen). Smoke points have also been determined during the addition of water vapour to a nitrogen-oxygen stream, over the same range of oxygen index values. The shape of the smoke point against oxygen index curves is surprisingly complex. Dilution also caused temperature and structural changes in the flame. In the light of these results the factors believed to be of relevance to soot production are discussed. The smoke point values were compared with previously recorded values, and appear to depend on variables such as burner design.