Distearoyl-sn-glycero-3-phosphocholine (DSPC) and polyoxyethylene( 40)stearate (PEG 40 St) were investigated with Langmuir monolayer isotherms and Brewster angle microscopy (BAM) imaging at DSPC/PEG 40 St molar ratios ranging from 9:1 to 5:5. Two plateaus were found in the Langmuir isotherms which were relatively shorter for the 9:1 mixture and extended significantly by increasing the PEG 40 St content, indicating that the PEG 40 St squeezed out whereas more emulsifier retained in the monolayer at higher PEG 40 St contents. A strong hysteresis was observed when the mixed monolayers were subjected to compression-expansion cycles. The degree of hysteresis for the first cycles also increased with increasing PEG 40 St content in the monolayer. Gray scale intensities in the Brewster angle microscopy images were determined for pure DSPC and pure PEG 40 St and a scale was established to better interpret the morphologies for the mixtures. Bud and vessels formed during the PEG 40 St squeezed out upon compression. Upon expansion, PEG 40 St and DSPC is reappeared on the monolayer. When considered BAM images together with the Langmuir isotherm, PEG 40 St molecules were found to be well distributed within the DSPC molecules at lower DSPC/PEG 40 St mole ratios and mostly phase separated at higher mole ratios. It was concluded that higher PEG 40 St content would be advantageous for the design of an efficient and cheaper ultrasound contrast agents. A lipid-to-emulsifier mole ratio of 9:1 is generally used for the microbubble composition [4,18,20,[24][25][26][27][28]. The phase formation, miscibility, and microbubble stability of this composition have still been under investigation. Emulsifier content less than 10% (9:1 mixture) is