The Acoustic Bubble Spectrometer has been identified as a potential method for monitoring the size distribution of aphrons in situ, such as in an oil well drilling fluid flowline.1 Research was continued from Task 1.1 of this Project, Aphron Visualization,2 in which ABS was tested against laser light scattering (Coulter Counter) and optical (visual) imaging to determine the bubble size distribution (BSD) of the aphrons at ambient temperature and pressure. Task 2.1 continued this investigation by measuring the bubble size distribution via ABS and optical imaging at elevated pressures up to 2000 psig.