<p>Poly(vinyl alcohol)/chitosan (PVA/CS) composite membranes containing 20 wt%_CS glycerol were prepared by solvent casting and cross-linked with 30 wt%_PVA sulfosuccinic acid (SSA) via hot pressing. This study investigates the combined effects of the addition of 5 wt%_CS kraft lignin (KL) or organosolv lignin&#xa0;(OL) and thermal cross-linking conditions (110&#xa0;°C for 2&#xa0;h or 120&#xa0;°C for 3&#xa0;h, at 25&#xa0;kg·cm⁻²) on the physicochemical and ion transport properties of the membranes. Although lignin is more hydrophobic than the PVA/CS blend, the incorporation of KL or OL disrupts the hydrogen-bonding network, leading to a less cohesive polymer structure and increased water uptake. Nevertheless, when the membrane undergoes cross-linking, lignin particles actively participate in forming a compact network that favours SSA retention, leading to higher gel content and enhanced resistance to swelling, thermal decomposition, and proton-transport-induced degradation, particularly under more severe conditions. In particular, the systems containing KL exhibit greater consumption of hydroxyl groups during cross-linking and a higher incorporation of sulfonic acid groups, thereby enhancing proton diffusion and proton conductivity. These findings demonstrate that integrating lignin coupled with a controlled thermal cross-linking is an effective strategy for tailoring PVA/CS composites for energy-related applications.</p> Graphical abstract <p></p>