Petrological analysis of the 2020–21 La Soufrière lava dome reveals ubiquitous oxidation textures. Comparison of the natural dome rock to subsequent explosive scoria phases highlights the lack of any oxidation features in the latter, indicating that oxidation processes affected only the dome-forming magma, either during pre-eruptive storage or upon emplacement. To investigate the causes of oxidation we present a series of one-atmosphere experiments, using fresh natural basaltic andesite scoria as a starting material. Experiments were performed at 900 and 1020°C and at oxygen fugacities between NNO − 2 and air. Experimental results show that iron oxide nanolites nucleate on the rims of pyroxene microlites and phenocrysts under all experimental conditions except at NNO − 2. Orthopyroxene phenocrysts become unstable at 1020°C, at and above NNO + 2. Olivine symplectites form in all experiments at and above NNO. Titanomagnetite co-exsolves titanohematite and an Mg–Fe–Al spinel (pleonaste–magnesioferrite) at and above NNO + 2. Well-developed Mg–Fe–Al spinel trellis exsolution lamellae in titanomagnetite phenocrysts, as seen in the dome, only form in the presence of air at 900°C. The combination of textures and compositions observed in the natural dome indicates that oxidation of the dome magma occurred during emplacement at Earth's surface, with air percolating through the dome at temperatures ≤900°C.