During explosive volcanic eruptions, pumice clasts are transported into the atmosphere, and their thermal and degassing histories determine how much volatiles are lost syneruptively. This process is studied by combining a steady state eruption column model with a model for the cooling and degassing of pumice. In the model we investigate the impact of various parameters (e.g., mass eruption rate, Biot number, eruption temperature, and geometry) on the cooling and degassing of pumice and find that the Biot number and the eruption temperature are the most influential. During typical risetimes of pumices inside eruption columns (200--300 s), those smaller than 0.5 cm in diameter are found to lose little of their volatiles syneruptively and remain in thermal equilibrium with the plume. In the case of larger pumices the ratio of the cooling timescale (∝rp2/&kgr;, where rp is the pumice radius and &kgr; is the thermal diffusivity) to the degassing timescale (∝&xgr;w2/D0', where &xgr;w is the vesicle wall thickness and D0' is the species diffusivity) controls degassing. If this ratio is larger than 50, degassing of the pumice will be nearly complete with a few percent of the original volatiles left in the outer rind of the pumice. For a ratio of less than 0.1 no volatiles escape the pumice. Typical values for thermal and species diffusivity as well as vesicle wall thickness indicate that in the case of degassing Cl and H2O the transition from 0.1 to 50 will be in the pumice size range of 1--10 cm. Here volatile concentration is a strong function of position inside the pumice, suggesting that values for volatile contents measured on picked matrix glass from crushed samples may not be the best way to estimate the volatile content of matrix glass, a number which is frequently used to estimate the total volatile input of volcanic eruptions. This effect is even more pronounced when looking at hydrogen isotopic fractionation with larger pumices tending toward ΔD-80?. ¿ 2001 American Geophysical Union