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Compartment fire experiments were performed using either natural or overhead forced ventilation to assess the effect of ventilation on the compartment fire environment. The ventilation rate was varied by changing the vent size in natural ventilation tests and mechanically in forced ventilation tests. Three fuels were used: diesel fuel, wood cribs, and polyurethane slabs. For naturally ventilated fires, it was noted that the vent size and geometry had a significant impact on the vertical temperature and oxygen concentration profiles. Both the temperature and oxygen concentration measurements were representative of two-layer environments. However, in terms of oxygen concentration, forced ventilation fires resulted in a well-mixed compartment regardless of the ventilation rate. In contrast, temperature measurements showed either a two-layer or linear variation with respect to compartment height, depending on the scenario. These results defy the conventional assumption that compartment temperatures and gas concentrations have the same vertical distribution. Furthermore, these results identify the need to go beyond the classic two layer paradigm. In addition, reduced oxygen concentrations at the flame base resulted in reduced mass loss rates. The relationship between these burning rates and the oxygen concentration at the flame base was examined, and a correlation was developed. The results show that temperature and flashover potential assessments based on furniture calorimeter data may be significantly overestimated.