Explosive volcanic eruptions frequently generate fall and flow deposits simultaneously, which can be attributed to a marginally stable atmospheric column in transitional conditions between the buoyant and collapse regimes. This behavior is reproduced by laboratory experiments and numerical simulations. Ten well-documented eruptions are used to test theoretical models of explosive eruptions. Three types of deposits, fall, flow, and composite deposits made of intercalated flow and fall units, are observed in these eruptions. Estimates of mass discharge rate and initial volatile concentration in the magma are available for each eruptive phase. Using the simple assumptions that (1) the mass fraction of gas in the mixture is equal to the initial volatile content of magma and (2) jet expansion outside the vent is unconstrained by crater dimensions, theoretical predictions are not consistent with the data. Agreement between data and theory may be achieved by appealing to imperfect degassing of pyroclasts, which lowers the gas content of the erupted mixture. The effective amount of continuous gas phase carrying pyroclasts in suspension depends on the size distribution of pyroclasts. In coarse pyroclast populations a large amount of magmatic gas remains trapped in bubbles within the pyroclasts and is not involved in the bulk volcanic flow. A new regime diagram based on estimates of the effective gas content in the erupted mixture allows good agreement with the observations. For given mass flux and initial dissolved volatile content, changes of the size distribution of pyroclasts may have a strong effect on atmospheric column behavior. ¿ 2001 American Geophysical Union