To further develop the potential for carrageenan-use in a seaweed biorefinery, the acid-catalyzed pyrolysis of kappa-, iota- and lambda-carrageenans has been studied in polyethylene glycol (PEG). Using differential scanning calorimetry, it was found that decomposition temperatures for the three polysaccharides were broadened and lowered by 33–60 °C in the presence of H2SO4 and PEG compared to polysaccharide alone. Levoglucosenone (LGO) and furfural were the major products from all polysaccharides with LGO yields of 21.3 mg/gcarr (3.5 mol%), 10.7 mg/gcarr (2.2 mol%) and 3.9 mg/gcarr (1 mol%) and furfural yields of 5.9 mg/gcarr (1.3 mol%), 2.8 mg/gcarr (0.7 mol%) and 1.2 mg/gcarr (0.4 mol%), for kappa-, iota- and lambda-carrageenans, respectively. Total sulfate inversely correlated with the generation of volatile compounds, and the 3,6-anhydro structure may encourage the formation of the major products during the acid-catalyzed pyrolysis of carrageenan in PEG.