A mine fire e x c i t e s in t he ventilation network nonsteady-state gas-air flow with a high temperature, a reduced oxygen content, and a critical content of carbon monoxide. A change in air temperat~re is an additional thermal (fire) pressure. A fire pressure destabilizes the air distribution in the network and it creates conditions for slowing down and reversing air streams. As a result smoke enters new regions of the ventilation network and an there is an increase in methane concentration in some mine workings [3, 4, 9].