Abstract For flammable gases and liquids in air, the upper and lower flammable limits are adequate to define the flammability. However, for either nitrogen or oxygen enriched mixtures the flammability is expressed more generally as a boundary in a flammable zone. This work presents equations to model the boundary of the flammable zone, with an emphasis on developing simple, direct algebraic equations. Separate equations are developed for the upper and lower boundaries to estimate the flammable zone. Two methods are presented to predict the upper and lower flammability zone boundaries: a simple thermodynamic model and an empirical model. The flammability zone boundary models are also applied to predict the limiting oxygen concentration (LOC) and the upper and lower oxygen limits (LOL). The simple thermodynamic model is based on the adiabatic temperature change and assumes a simple reaction scheme and constant physical properties. This method approximates the boundaries as linear equations. This method uses the upper flammability and lower flammability limits (LFL) to calculate two adiabatic flame temperatures and attempts to account for the changes in the reactions along the upper flammability zone boundary. The empirical method approximates the boundaries with linear equations that are functions of the upper or LFL. Comparison of the models to experimental data shows that the empirical model works best for estimating the flammability zone boundaries. This model also estimates the limiting oxygen concentration, upper oxygen limit (UOL), and the LOL quite well. The regression coefficient values for the limiting oxygen concentration, UOL, and LOL are 0.672, 0.968, and 0.959, respectively. This is better than the fit of the y LOC = zy LFL method for the LOC, in which the regression coefficient's value is 0.416. © 2009 American Institute of Chemical Engineers Process Saf Prog, 2010