The effect of changes in the jet nozzle geometry, i.e., nozzle shape and lip thickness, on the blowout limits of jet diffusion flames in a co-flowing air stream were experimentally investigated for a range of co-flow air stream velocities. Circular and elongated nozzles of different axes ratios were employed. Preliminary results showed that nozzles with low major-to-minor axes ratios improved, while high ratios reduced, the blowout limit of attached flames compared with that for an equivalent circular nozzle. The nozzle shape had no apparent influence on the blowout limits lifted flames and the limiting stream velocity. The experimental blowout limits of lifted flames were found to be a function of the co-flowing stream velocity and jet discharge area. On the other hand, the stability of attached flames was a function of the co-flowing stream velocity, jet discharge as well as the nozzle shape. The effect of premixing a fuel with the surrounding air was also studied. Generally, the introduction of auxiliary fuel into the surrounding stream either increased or decreased the blowout limit depending on the type of flame stabilization mechanism prior to blowout. The stability mechanism of the flame was found to be a function of the co-flow stream velocity and the auxiliary fuel employed.