Southern Ocean organisms are thought to be particularly vulnerable to ocean acidification, as they inhabit cold waters where calcite-aragonite saturation states are naturally low. It is also generally assumed that calcifying animals would be more affected by ocean acidification than non-calcifying ones. In this context, we aimed to study the impacts of reduced pH on the ascidia Cnemidocarpa verrucosa sp. A. Here, we used gene expression profiling and enzymatic activity to study the responses of that Antarctic benthic species to ocean acidification. We sampled Cnemidocarpa verrucosa sp. A. by scuba diving at approximately 15 m depth at Carlini station, Potter Cove, King George Island, Antarctica. Superoxide dismutase (SOD) activity was measured in the ascidia, samples (approximately 70 mg of brachial basket) were homogenized in 20 mM Tris-HCl, 1 mM EDTA, pH 7.6, with a ratio 1:4 w/v. Homogenates were centrifuged at 14,000 x g for 3 min at 4°C and the supernatant was used to measure SOD activity at 20°C following Livingstone et al. (1992) protocol. Supernatant was mixed with the measurement buffer (43 mM K₂HPO₄, 43 mM KH₂PO₄, 0.1 mM EDTA, pH 7.68), 5 mM Xanthina (Sigma X-0626), 100 µM Citocromo-C (Sigma C-2037), 0.3 mU/µl XOD (Xanthin-Oxidasa, Sigma X-4875) in 2 M (NH₄)2SO₄. The measurement was made in a photometer at 20°C, 550 nm wavelength, for 3 minutes, every 10 seconds. For the calculations, the total protein content of the samples was measured using the method of Bradford (1976). Superoxide dismutase activity was expressed in activity in the extract (mU) / amount of protein (mg). All measurements were made in triplicate.