A detailed study is presented on the synthesis and characterization of purely oxygen-filled microbubbles (OMBs) stabilized by phospholipids. Microbubbles with a diameter of less than 10 pm were generated and concentrated to >50 vol % in saline. The lipid acyl chain length had little effect on the size distribution but profoundly affected the foam stability. For example, OMBs stabilized by dipalmitoyl phosphatidylcholine (DPPC) degraded over 3 weeks, but OMBs stabilized with distearoyl phosphatidylcholine (DSPC) retained over half of their initially encapsulated gas. Interestingly, the polydisperse size distribution remained nearly constant as the foam slowly broke down. Injection into an undersaturated solution led to the immediate release of the oxygen gas core. Injectable gas delivery by OMBs may find use in a variety of medical and industrial fields.