Abstract <p>The paper reviews the results of laboratory investigations into fragmentation of highly viscous magma (porous dacite samples from the cryptodome of Mount St. Helens) by rapid decompression. Setups for experimental study of fragmentation of analog materials and natural samples of highly viscous dacite magma under rapid decompression (overpressure up to 200 bar and temperature up to 1000°C) are discussed. The fragmentation thresholds (critical pressure differences between included bubbles and surrounding magma that lead to fragmentation of a pressurized sample) were determined for porous dacitic samples. The threshold pressure contrasts were found to range between 90 and 30 bar (with dacite temperature changing from 15 to 900°C). The fragmentation rate of porous dacite samples, <i>N</i>, was estimated to be 5–170 m/s. It increased with pressure differential ∆<i>P</i> and decompression rate. The study of the fragmentation products has shown that dacite samples fragment at a temperature of 900°C exclusively brittlely. The experiments conducted at temperatures <i>T</i> = 20°C and 900°C with overpressure ∆<i>P</i> in the range 10–185 bar have shown that with increase in overpressure ∆<i>P</i> and decompression rate, the characteristic size of particles formed by fragmentation decreases. From the experiments conducted at ∆<i>P</i> = 120 bar and a temperature <i>T</i> = 15–900°C it was derived that increase in temperature <i>T</i> hinders fragmentation (fragmented ejecta become larger) and leads to bimodal particle size distribution.</p>