The transition from a plinian (pumice) to an effusive (dome-forming) eruptive style is frequently observed in volcanic systems and is generally attributed to the progressive loss of volatiles from magma stored in a superficial reservoir. This explosive-effusive transition has been explained by the evolution from a dosed to an open system of degassing(1-4). But in this context, an eruption at Mt Pelee (Martinique, French West Indies) dated at 650 years ago, which exhibited a rarely observed(5,6) succession from dome-forming to plinian activity in a short interval of time(7), is at odds with such an explanation. In this eruption, near-surface explosions of the dome produced two peleean turbulent pyroclastic flows, whose deposits are similar to those of the effusive 1902 eruption, and then plinian activity produced pumice fallouts and flows. The reconstruction of the degassing paths of both eruptive regimes using the densities and the H2O and Cl contents of the clasts shows that the interaction of rising magma with hydrothermal fluids at shallow depth may play a critical role in determining eruptive style.