A Mine Fire Diagnostic (MFD) Methodology was developed to determine the location and extent of combustion zones in abandoned underground coal mines. In this method, a characteristic fire signature is based on the ratio of higher molecular weight hydrocarbon gases (C-2 to C-5) to total hydrocarbon gas. Initially, gas samples are obtained at the bottom of boreholes under baseline or static conditions. A second set of samples is obtained when a suction fan is used to influence the direction of gas movement. Pressure data define the degree of communication between boreholes. The value of the diagnostic ratio under communication conditions is taken as a measure of subsurface fire activity related to a particular flow direction. Using a Venn diagram technique, the results are mapped as quadrants on a borehole map of the site. Repetition of the communication tests provides overlapping quadrants that define hot, cold, and indeterminate areas. The MFD has been used to distinguish hearted and cold subsurface areas at four mine fire sites. At each of the sites, the extent of the fire could not be inferred from surface evidence, and the location of combustion zones had a significant impact on plans to control the fire. Although the method is labor intensive and requires drilling cased boreholes, it is relatively simple and provides information that cannot be obtained by other methods. (C) 2004 Elsevier B.V. All rights reserved.