The 5 April 2003 paroxysm of Stromboli occurred during an ongoing effusive episode that initiated on 28 December 2002. It mainly consisted in two, a few seconds apart cannon-like explosions followed by a vertical gas/pyroclast jet. The HK-basaltic pumices, that were generated, mainly contain crystals that were drained back from the shallow parts of the plumbing system. Few euhedral to anhedral olivines and their melt inclusions testify to crystal fractionation and possible mixing between HK-basaltic magmas variable in their volatile content, extent of evolution and volume. The most primitive, volatile-rich, HK-basalt term is sizable at micrometer scale, only. We propose that the triggering mechanism of the paroxysm was initiated at moderate pressures (≥240 MPa) and was related to bubble-driven ascent of magma blobs. The latter rose through and interacted with overlying slightly more evolved melts and, finally mingled with the shallow crystal-rich magma just before the eruption.