This thesis describes the details used to model a post-flashover fire compartment as a well-stirred reactor. In particular, it examines the two foremost important variables that dictate post-flashover fire behaviour inside the fire compartment. These two variables are: (1) the mass flow rate of air into the compartment via the vent opening and (2) the fuel mass loss rate inside the compartment. The vent flow analysis shows that the orifice analogy typically used to describe compartment vent flow is restricted to small wall opening applications. For large wall openings, such as a window occupying one whole wall, the flow rate is dictated by the plume entrainment with a flow rate -60% of the flow rate estimated from the orifice theory. A series of fire experiments using a reduced-scale compartment were conducted to study the vent flow behaviour in a compartment with a horizontal roof opening and a vertical wall opening. Based on the analytical and experimental studies, it is shown that in the case where the roof vent opening is not excessively large and a wall opening having a small downstand, the neutral-plane exists below the soffit of the wall opening giving outflow and inflow through the wall opening and outflow through the roof opening. In such a case, the flows through these openings can be adequately described using an extended form of the vent flow formulation that includes the roof vent opening. The area of the roof vent and the depth of the downstand between the ceiling and the soffit of the wall opening are found to be significant in determining the extra air inflow induced due to the existence of the roof vent opening. The cellulosic and pool fuels each have different burning behaviour inside a compartment. However, compartment fire temperatures and fuel mass loss rates, from both fuel types, are strongly dependent on the available fuel surface area to ventilation opening ratio and the fuel surface to enclosure area ratio. In the case of thermoplastic pool fires, the ratio between the heat of combustion of air for the fuel and the heat of gasification of the fuel is also found to be influential on the resulting fires. A post-flashover fire computer program, CFIRE, has been developed that incorporates these findings. The simulation studies performed using the CFIRE computer program show that the fire time-temperature histories of wood fires are highly dependent on the remaining fuel surface area over time. In the case of thin wood, a shorter and hotter fire is expected as it has greater surface area than thick wood, even with the same fuel load. The study also shows that for small ventilation opening, a pool fire inside a fire compartment is less severe than a wood fire because the thermoplastic fuel is easily vaporised under the radiation feedback from the hot surrounding environment and discharged outside the compartment. In the case of large openings, pool fires are more likely to produce a hotter fire in the compartment than wood fires because wood fuel would not have sufficient fuel surface area to achieve ventilation controlled burning. By comparing these simulated fires with the Eurocode parametric fires, the Eurocode parametric fires do not provide realistic decay rate. With the modified parametric fires, these fires are conservative as they provide envelopes for the simulated fire curves. However, these parametric fires do not clearly describe the fire behaviour of realistic furnishing inside the fire compartment.