The applicability of using the oxygen index test (ASTM D 2863-76) to obtain an indication of the relative flammability of fire-retardant- treated wood products was investigated. The oxygen index is the minimum percentage oxygen that is required to maintain flaming combustion of a specimen under specified laboratory conditions. Within the plastics industry, the test is used in research and development and in quality control. Since the oxygen index test offers a single numerical value and requires relatively small specimens, it was desirable to determine its applicability to wood products. Specimens from nine boards of southern pine were tested to obtain oxygen index values for untreated wood and wood at two treatment levels of diammonium phosphate. Next, untreated southern pine specimens were tested for the effects of grain direction, MC, and thickness of specimen. Finally, specimens from a single sheet of Douglas- fir plywood were treated with eight chemicals at four treatment levels and tested for oxygen index. The plywood oxygen index results were compared with available data for the fire tube, modified Schlyter, and 8-foot tunnel tests. The results show that the oxygen index test can be used as an indication of the flammability of a fire-retardant-treated wood sample relative to other fire-retardant-treated and untreated wood products, the results for the untreated and treated samples showed a range of 22 to 78 and an average coefficient of variation of 3 percent.