Abstract We apply dynamical models to estimate the rheological properties of magma and lava during the 2021–22 eruption of La Soufrière Volcano, St Vincent. Analysis of the emplacement of a lava coulée gives viscosities in the range 0.94 × 10 10 Pa s to 5.97 × 10 10 Pa s. A static Bingham model gives a yield strength of 4.1 × 10 5 Pa. A dynamical model of conduit flow during the explosive phase of the eruption gives a viscosity range from 1.2 × 10 7 to 2.3 × 10 9 Pa s. A petrological model of magma viscosity in the source region falls in the range 10 2 to 10 3 Pa s. The very high viscosity of the lava is attributed to the presence of a remnant degassed and partially crystallized magma from previous eruptions that occupied the shallow conduit–vent system prior to onset of the 2021 explosive eruptions. Gas-rich magma pushed this degassed remnant magma out over a three-month period at a steady rate of about 1.2 m 3 s −1 and the explosive phase began when volatile-rich and much lower viscosity magma reached the surface.