We performed experiments to constrain the effects of sulphur and oxygen fugacities on magma chamber and outgassing conditions of the La Palma 2021 eruption. Based on a series of controlled experiments on basanitic products carried out at 1040 °C and 200 MPa, we show that sulphur addition affects the stabilities of amphibole and olivine, in particular at high fO₂ and elevated S contents which together inhibit amphibole crystallization. We also show that the overriding control on S systematics is oxygen fugacity, with melts capable of dissolving from 1000 up to 8000 ppm S, depending on fO₂. Increasing the bulk S content increases the S content of the silicate melt up to ∼2000 ppm for fO₂ < NNO + 2, and 7000–8000 ppm at higher fO₂. Further increase in dissolved S is prevented by the buffering effects of either sulphide at low fO₂ or anhydrite at high fO₂. Modelling shows that the observed CO₂/SO₂ and H₂O/SO₂ ratios of volcanic gas emissions during the eruption imply a pre-existing >5 wt% exsolved fluid in the reservoir, with fS₂ at ∼0.1 MPa at fO₂ above NNO. Our work confirms that basaltic magmas may coexist with a significant amount of excess fluid which in turn holds an important part of the sulphur budget emitted to the atmosphere.