Ocean acidification (OA) and the associated changes in seawater carbonate chemistry pose a threat to calcifying organisms. This is particularly serious for shelled molluscs, in which shell growth and microstructure has been shown to be highly sensitive to OA. To improve our understanding of the responses of abalone to OA, this study investigated the effects of CO 2-induced ocean acidification on extra-cellular acid-base parameters in the European abalone Haliotis tuberculata. Three-year-old adult abalone were exposed for 15 days to three different pH levels (7.9, 7.7, 7.4) representing current and predicted near-future conditions. Haemolymph pH and total alkalinity were measured at different time points during exposure and used to calculate the carbonate parameters of the extracellular fluid. Total protein content was also measured to determine whether seawater acidification influences the composition and buffer capacity of haemolymph. Extracellular pH was maintained at seawater pH 7.7 indicating that abalones are able to buffer moderate acidification (-0.2 pH units). This was not due to an accumulation of HCO 3-ions but rather to a high haemolymph protein concentration. By contrast, haemolymph pH was significantly decreased after 5 days of exposure to pH 7.4, indicating that abalone do not compensate for higher decreases in seawater pH. Total alkalinity and dissolved inorganic carbon were also significantly decreased after 15 days of low pH exposure. It is concluded that changes in the acid-base balance of the haemolymph might be involved in deleterious effects recorded in adult H. 2 tuberculata facing severe OA stress. This would impact both the ecology and aquaculture of this commercially important species.