The May, 2008, Chaitén (southern Chile) eruption was characterized by several explosive events, each associated with plumes which reached up to about 19 km above sea level on May 6. A study of the textural and physical features of the juvenile clasts erupted during the climactic phase of the 2008 eruption of Chaitén is presented. Pumice clasts show unimodal density distribution (main mode at 600 kg/m3), average vesicularity of about 69 %, a glassy groundmass with no microcrystals, and vesicles with dimension between ∼1 μm and ∼2 mm. They also show a unimodal vesicle size distribution with most frequent vesicle size in the range 0.05–0.08 mm and an estimated vesicle number density of 1.3±0.5×105mm−3 related to a rapid nucleation event produced during the late phases of magma rise. This is confirmed by the absence of microcrystals that could otherwise have delayed vesicle formation and allowed the magma to maintain a low viscosity and a supersaturation in volatiles. Vesiculation and fragmentation were triggered by a sudden decompression of the melt associated with the opening of the volcanic conduit (∼10 MPas−1).