Abstract: The instrument tests specimens by means of the critical oxygen index method and comprises a twin-walled chimney of transparent material in which the specimen is mounted. A helically wound heater coil is positioned between the chimney walls to heat the interior of the chimney and a preheater is provided to enable gases entering the chimney to be heated, the combination of heaters enabling a closely controlled temperature gradient to be maintained around the specimen. The specimen is tested by introducing into the chimney a variable mixture of nitrogen and oxygen and igniting the specimen under controlled conditions.