We have endeavored to demonstrate theoretically and experimentally that, in the inhomogeneous combustion of hydrocarbon-air systems, a high degrce of similarity prevails in the distribution of the initial fuel (H and C) and oxidant (O and N) elements, respectively—subject to the limitation that free soot does not escape from the flame. The same data were used to establish the effective mean diffusion coefficient of the elements in ethane-air flames, ( D 1 cm 2 /sec=0.23±10 per cent) and butane-air flames ( D 1 cm 2 /sec=0.18±10 per cent). There is good reason to believe that this same range of values holds for the inhomogenous combustion (with air) of any hydrocarbon, regardless of its molecular weight. The data were treated in terms of analytical expressions which, theoretically, are valid in the presence of gradients and variations in the mean molecular weight of the gases. The butane-air flame analysis strongly supports this theoretical prediction. On the basis of mixing similarity, a picture was developed which is in accord with a wide variety of observations on both the gross and detailed structure of laminar diffusion flames, and which provides a common physical basis for the work of Burke and Schumann, of Jost, and of Wolfhard and Parker. Mixing similarity clearly delineates the domains of diffusional, chemical kinetic, thermo Supplementary information (Tables similar to 1A and 2A, appendix, for all flames) has been deposited as Document Number 4981 with the ADI Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. A copy may be secured by citing the Document number and by remitting $2.50 for photoprints or $1.75 for 35-mm microfilm. Advance payment is required. Make checks or money orders payable to: Chief, Photoduplication Service, Library of Congress. dynamic and fluid dynamic phenomena. Although, for reasons of mathematical simplicity and illustrative clarity, we have dealt entirely with laminar diffusion flames, the general physicochemical principles are applicable to any flow regime.