Decomposition pathways of detritus are key processes in the contribution of macroalgal habitats to natural carbon sequestration of ‘blue carbon’. The anaerobic decomposition of 3 North-east Atlantic canopy-forming kelp species was investigated using ex situ decomposition chambers. Thallus parts (stipes, holdfasts and blades) of Laminaria hyperborea, Saccharina latissima and L. digitata were incubated in still seawater in temperature-controlled dark conditions. Refractory potential (Rp), first-order decomposition rate (k) and associated half-life (t1/2) were calculated. Dissolved organic and inorganic carbon (DOC and DIC, respectively) were measured in the incubation water at 0, 7, 14 and 21 d, and thermal gravimetric profiles were determined at each decomposition stage. Oxygen depletion occurred within 24 h. Approximately 5 times as much DOC was released than DIC. Holdfast material produced the most DIC, while blade material released the greatest amounts of DOC. S. latissima released less DOC than L. hyperborea and L. digitata. The mean (SD) Rp of fragments increased from 0.46 ± 0.05 to 0.50 ± 0.04 throughout the 21 d incubations. S. latissima had the highest Rp throughout. First-order decomposition rates, averaged across the 3 kelp species, gave half-lives (t1/2) for blade fragments of 27.8 ± 2.9 d, (k = 0.025 ± 0.002) and stipes as 113.2 ± 21.1 d (k = 0.006 ± 0.001). This experiment clarifies the fate of macroalgal carbon during early decomposition and thus the processes that govern blue carbon pathways of macroalgae, highlighting the differences in breakdown among different species and thallus parts.