The mathematical treatment relating to mixture of fluids in a container maintained at constant pressure and volume is developed, assuming perfect and instantaneous diffusion, free efflux and no reaction between the components of the mixture. For comparison, the equations for mixture within a container from which no efflux takes place are given and also a treatment applicable to intermediate conditions. Specific applications are developed pertaining to conditions involving inert gas, such as carbon dioxide, flue gas or engine exhaust gas, released into an inclosure by automatic or manual means to extinguish fire, and to the subsequent ventilation of spaces thus deluged to reduce the toxic gas content and raise the oxygen content to limits making such spaces safe for entry.