The anaerobic digestion (AD) of municipal biopulp with two macroalgal biomasses (i.e. Saccharin latissima and Focus serrates) was investigated at batch and continuously fed digesters at thermophilic conditions (54 +/- 1 degrees C). At batch mono-digestion tests, municipal biopulp was associated with significantly higher methane production (549 +/- 9 mLCH(4)/gVS) compared to both S. latissima (210 +/- 13 mLCH(4)/gVS) and F. serrates (206 +/- 37 mLCH(4)/gVS). Regarding batch co-digestion tests, the highest methane yield was achieved when the feedstock consisted of 80% VS of biopulp and 20% VS of macroalgal biomass and it corresponded to the single methane contributions. The batch results were confirmed by continuous mode operation experiments, for the mono-digestion of biopulp and subsequently, the co-digestion with S. latissima. A specific challenge encountered with macroalgae biomethanation is the high sodium content. Therefore, mathematical modelling was followed to predict the performance of continuous mode experiments under increased salinity conditions by simulating the addition of more saline feedstock. The experimental results were used to calibrate and validate the model. Modelling simulations revealed that usage of saline feedstocks can drastically inhibit a well-performing AD reactor.