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The 5-day biochemical oxygen demand (BOD 5 ) test has been used for over a century to evaluate the concentration of biodegradable organic matter in wastewater effluents. The BOD test is a simple batch test, but it is also time consuming, labor intensive, imprecise, and, due to the dilution of the wastewater, takes a relatively long time to complete. Advances in laboratory instruments and lab ware have made it possible to improve tests to measure oxygen demands of samples. Respirometric techniques are now widely used in Europe and are proposed for inclusion in Standard Methods. The headspace BOD (HBOD) test is a batch respirometric method that was developed only seven years ago. The test overcame many limitations of the BOD test, but its main shortcoming was that it was difficult to rapidly measure oxygen in the headspace of samples. We report here a new method to directly measure oxygen in the headspace of HBOD tubes using a fiber optic probe we refer to as an HBOD probe. We demonstrate that the HBOD probe provides data identical to that obtained with a gas chromatograph, but that sample analysis takes only minutes. Field tests conducted at two local wastewater treatment plants show that HBOD values are obtained in only 2 to 3 days that are similar to the BOD 5 values. We also demonstrate that HBOD values are more precise than those obtained in the BOD test.