The viscosity of three molten basalts produced by fusion of lavas issued from different eruptions of the Piton de la Fournaise (La Reunion island) has been investigated above the liquidus and at high degrees of supercooling. For all basalts. crystallization took place rapidly above the glass transition range, which made static measurements problematic because of time-dependent, non-Newtonian rheology. By contrast, reproducible results were obtained in dynamic measurements made at a constant heating rate of 5 K/mn. Partial crystallization resulted in strong viscosity increases which were primarily due to the presence of solid inclusions. Comparisons with phase equilibria experiments performed for the same samples at higher temperatures show that crystallization takes place differently below the liquidus and at strong degrees of supercooling. As recently described for simpler systems, these differences in nucleation and growth are controlled by the relative mobilities of the cations diffusing within the melt to attach themselves to crystal-melt interfaces. A consequence is that the composition evolution of the residual melt differs in equilibrium and irreversible crystallization. In turn, this contrast induces opposite viscosity variations for the residual melt. (c) 2008 Elsevier B.V. All rights reserved.