Abstract: An apparatus (9) for testing the efficiency of a thermal oxidizer (200) is described. The apparatus provides for a test chemical to be directed into a thermal oxidizer so that the entry and exit flow rates of test chemical through the termal oxidizer may be accurately measured. In a portable version, the apparatus is formed of a platform (10), a tank (26) for holding liquid test chemical to test chemical vapor, and a device (94) to measure the flow rate that liquid test chemical enters the vaporizer from the tank. Methodology employing the apparatus, which provides numerical data useful in determining the efficiency of a thermal oxidizer, is described.