Span-type surfactants (sorbitan fatty acid esters) and Tween-type surfactants (sorbitan polyoxyethylene fatty acid esters) are employed by us to generate stabilized microbubbles for use in diagnostic ultrasound. After sonication of an aqueous surfactant solution, only some mixtures of Span-type and Tween-type surfactants at certain conditions can form stable microbubbles. This work investigated the stability of the surfactant-stabilized microbubbles by using a Langmuir trough to measure the pi-A isotherms of the surfactant monolayer. The experimental results, which agreed with a theoretical analysis of the microbubble stability, indicate that the surfactant-stabilized microbubbles have a solid-condensed monolayer "skin" which functions to reduce the surface tension, prevent coalescence between microbubbles, and increase their aqueous compatibility. The higher surface pressure obtained for the case of a microbubble preparation, compared with that of unsonicated mixtures, indicates that sonication enhances the structure of surfactant monolayer and makes the microbubbles extremely stable.