The spatial distributions of the hydroxyl radical (OH), formaldehyde (H2CO), and temperature imaged by laser diagnostic techniques are presented using a Jet in Hot Coflow (JHC) burner. The measurements are of turbulent nonpremixed ethylene jet flames, either undiluted or diluted with hydrogen (H-2), air or nitrogen (N-2). The fuel jet issues into a hot and highly diluted coflow at two O-2 levels and a fixed temperature of 1100 K. These conditions emulate those of moderate or intense low oxygen dilution (MILD) combustion. Ethylene is an important species in the oxidation of higher-order hydrocarbon fuels and in the formation of soot. Under the influence of the hot and diluted coflow, soot is seen to be suppressed. At downstream locations, surrounding air is entrained which results in increases in reaction rates and a spatial mismatch between the OH and H2CO surfaces. In a very low O-2 Coflow, a faint outline of the reaction zone is seen to extend to the jet exit plane, whereas at a higher coflow O-2 level, the flames visually appear lifted. In the flames that appear lifted, a continuous OH surface is identified that extends to the jet exit. At the "lift-off" height a transition from weak to strong OH is observed, analogous to a lifted flame. H2CO is also seen upstream of the transition point, providing further evidence of the occurrence of preignition reactions in the apparent lifted region of these flames. The unique characteristics of these particular cases has led to the term transitional flame. (c) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.