In Part 1, the entrainment in the lowest regions of buoyant fire plumes burning from an environment of quiescent fresh air (lower layer) across an interface into an environment of hot combustion products (upper layer) is investigated. Measurements using 20 to 200 kW natural gas flames formed above a 0.19 m diameter burner show that the entrained mass flux is nearly linear with distance from the fire source and essentially independent of the fuel flow rate. Comparison with previous results suggests that this linear dependence is valid over a wide range of conditions, but that the magnitude of the entrainment near the burner is influenced by the initial buoyancy of the plume. The chemistry of the products in the upper layer is also investigated. Over the ranges studied, the composition was a function of the upper-layer equivalence ratio only, and independent of the temperature of the upper layer or the residence time of the gas in it. For fuel-lean fires the product composition suggests that the fuel reaction is nearly complete, in contrast to fuel-rich fires where some of the stable intermediates are found to "freeze out" of the reaction prior to completion. The experimental method is extended to entrainment measurements on steady, axisymmetric, fully turbulent jet diffusion flames of hydrogen in air. In the momentum-dominated regions of the flame, the nondimensional mass flux used to characterize this regime is found to be constant as expected. The transition between momentum-dominated and buoyancy-dominated regions is also observed. In Part 2, a novel diagnostic technique, which makes use of laser light scattered by soot particles, was used in an effort to identify flame sheets within a natural gas diffusion flame. Soot particles, inherently created and consumed in the flame, were used as the scattering medium, which obviated the need for externally supplied seed material. Since no foreign material was added to the flame, the current technique can be considered truly nonintrusive. The soot distribution within a large buoyant natural gas diffusion flame is argued to be a reasonable marker for the presence of a diffusion flame. Measurements made in 47.4 to 190 kW natural gas flames stabilized on a 0.5 m diameter burner show that when soot is present within the outer boundary of the flame, it is observed as thin sheets, which become narrower in regions where the average strain rate is estimated to be greater. The structure of the soot distribution suggests that the combustion occurs along sheets of diffusion flamelets that are highly wrinkled and convoluted. Furthermore, they are distributed fairly uniformly within the volume of the flame, based on images of the associated soot, and occupy about 40 to 60% of the volume.