A Mixture fraction analysis is performed to investigate the Characteristics of chemical species production in smoky compartment fires burning condensed-phase hydrocarbon fuels. A series of fire experiments were conducted in a 2/5 scale compartment based on the ISO-9705 room. Gas species and soot measurements were made at two locations in the upper layer of the compartment. The fuels considered, heptane, toluene, and polystyrene, generate highly smoky fires over a range of natural ventilation conditions. The mass fractions of measured chemical species, Such as unburned hydrocarbons, carbon monoxide, carbon dioxide, oxygen, and soot are presented as a function of mixture fraction and compared with state relationships based on the idealized reactions of a hydrocarbon fuel. The results show that plotting the local composition as a function of the mixture fraction adequately collapses hundreds of species measurements from an assortment of compartment conditions with varying heat release rates and spatial locations into a few coherent lines or bands. It is also shown that about half (or more) of the fuel carbon may exist in the form of carbonaceous soot in the upper layer of smoky compartment fires. Inclusion of soot in the mixture fraction analysis allows identification of fuel-rich or under-ventilated conditions. Published by Elsevier Inc. on behalf of The Combustion Institute.