In order to diversify the feedstock of raw materials for anaerobic digestion, macroalgae were investigated as a potential biomass. Particularly, Palmaria palmata and Saccharina latissima were tested for biogas production in semi-continuous reactors. Palmaria palmata permitted a better production of methane than Saccharina latissima (320 mL CH4 gVS-1 vs 270 mL CH4 gVS-1). After a long period of operation (182 days), a decrease in reactor performances occurred above a specific loading rate of 408 gVS kgVSS-1d-1. This decrease was attributed to inhibition by potassium and sodium and these results were confirmed by additional batch experiments. Indeed, the initial methane production rate was reduced by 53% when potassium and sodium concentrations reached a concentration of 13.0 g L-1 for potassium and 4.1 g L-1 for sodium. It corresponds to the ones observed at semi-continuous reactor failure.