The increasing demand for a low-cost and renewable carbon fibre precursor has driven the focus on bio-based precursors. Cellulose-lignin composite fibres are a new approach toward this direction. The combination of cellulose and lignin into a composite fibre could solve some of the current limitations for pure cellulose and lignin fibres. This study investigated the treatment of the composite fibres with boric acid with focus on carbon yield, stabilisation rate and fibre fusion, which is a typical defect in carbon fibre production. The influence of boric acid on the mechanism of stabilisation was studied. The stabilisation time was reduced by 25% through treatment with the reduction of fibre fusion, while the carbon yield increased significantly in comparison to the untreated fibres.