In order to avert hydrocarbons, (HCs) and some hydrofluorocarbons, (HFCs) inherent risk of fire, combinations with inert refrigerant fluids have already been investigated and accepted as the preferred approach for the replacement of hydrochlorofluorocarbons (HCFCs). Indeed, many of the HFCs offer characteristics (in particular, thermodynamic properties, performance, compatibility with existing equipment, moderate toxicity and relatively low cost) similar to the those of HCFCs. However, the mixing or use of a multiple-component blend may introduce a potential fractionation behavior, which cannot be disregarded due to its influence on the blend flammability characteristics. Assessing the evolution of the Concentration Limits of Flammability of a blend formulation as it is depleted is therefore essential and leads to the definition of the Flammability Ratio (FR) as the ratio of the concentrations of the flammable to nonflammable blend components. Only one FR value corresponds to one particular blend formulation. As flammability tests on different blend formulations (different FR values) are performed, one can define the threshold between flammable and nonflammable blend formulations. Per definition, the Critical Flammability Ratio (CFR) corresponds to the ER threshold between flammable and nonflammable blend formulations. This value sets a decisive baseline for fire risk assessment of refrigerant blends subject to fractionation behavior. This paper presents several methods to evaluate FRs and ultimately CFRs of a blend of multiple flammable and nonflammable components. As the FR value is varied until flammable and nonflammable blend formulations are found within a predetermined accuracy, new blend formulations are calculated and tested for combustibility. Relationships and computations from an initial blend formulation to another are therefore provided herein. Three different sets of assumptions to solve the required set of (n) equations system (for n components in the blend) are presented. Convergence, physical interpretation, extents and limitations of each method are included. (C) 2000 Elsevier Science Ltd. All rights reserved.