Acoustic agglomeration (AA) of the fly‐ash particles from coal combustion in pressurized, fluidized bed combustors for power plants is an intermediate treatment of the aerosol to improve the fractional removal efficiency of conventional particle removal devices such as cyclones so that recently proposed clean air requirements can be met. Tests were performed on Penn State's high‐pressure (10 atm), high‐temperature (891 °C) acoustic agglomerator (6‐in. i.d., 14‐ft length) at 158 dB and about 850 Hz, with particle loading of 10–18 g/m3and residence times of about 4 s. The sound source was a high‐efficiency siren of Penn State's design providing about 1500 acoustic watts. The mass fraction in the critical 0.5‐ to 5‐μm particle size range was reduced by a factor of 5 and the 10‐ to 20‐μm size range increased by a factor of 3. An acoustic agglomeration simulation model (AASM) computer code that predicts AA quite reliably at both atmospheric and HTHP conditions was developed at Penn State. The simulation results in terms of particle size distribution agree quite well with the test results from both test conditions. The simulation also predicts that better AA could have been obtained at higher frequencies (2000 Hz) and higher acoustic intensities (160 dB). [Work supported by DOE.]