An analytical model has recently been developed for predicting the circuit oxygen levels in semi-closed underwater breathing apparatus at various mission depths and diver activity levels (Nuckols, Clarke, Marr, 1999). Unmanned testing in June 2000 with a commercially available US Divers Aqualung OxyMix rebreather (NEDU Test Plan Number 00-06 dated May 2000) showed good agreement with this analytical model over a wide range of diver depths, orientations and activity levels (Nuckols, Gavin, Finlayson, 2000). However, the model indicated that an enhanced performance capability could possibly be achieved with the OxyMix by modifying the size of its exhaust bellows and changing the oxygen content of the supply gas. Following the tests in June, an optimization approach was applied to the analytical model to define desired design parameters for the OxyMix rebreather to maximize its excursion depth capability. Optimized exhaust volume ratios (EVR), the ratio of the small inner bellows volume relative to the main breathing bellows, and supply gas oxygen concentrations were identified to maximize safe operations for the OxyMix design. US Divers agreed to fabricate two modified exhaust bellows of varying volumes for testing to verify the predicted performances of the modified OxyMix rebreather. During the week of 18 December 2000, unmanned testing with these modified OxyMix rebreathers at the Experimental Dive Facility at NEDU verified predicted performances. Specific findings in these tests included a) a modified OxyMix rebreather using an exhaust volume ratio of approximately 6.25% and a supply gas mixture containing 37% oxygen was shown to give an excursion capability to 130 FSW while maintaining circuit PO/sub 2/ levels below 1.32 Ata (excursions to 150 FSW were shown to be possible with maximum PO/sub 2/ levels below 1.6 Ata) b) similar to the results seen in the June tests, circuit PO/sub 2/ levels were shown to be unaffected by diver activity rates between RMV's of 22.5 LPM and 62.5 LPM c) the addition of a diluent injection system on the modified OxyMix design provided minimum circuit oxygen levels of 0.42 Ata at 20 FSW and 0.25 Ata at 0 FSW while still maintaining circuit PO/sub 2/ levels below 1.32 Ata for excursions up to 130 FSW.