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The effect of sodium on the formation of methane from acetate in granular sludge from a UASB reactor has been determined at various acetate concentrations and pH-levels. At neutral pH sodium concentrations of 5, 10 and 14 g Na+l−1caused 10, 50 and 100% inhibition respectively, relative to the maximum specific acetoclastic methanogenic activity of the granular sludge. These values reflect the sensitivity of Methanothrix sp. towards sodium, as this is the predominant acetoclastic methanogen in the granular sludge used in this study. The pH did not affect the inhibitory action of sodium significantly in the range 6.5–7.2, but at pH-levels near 8 the inhibition was more pronounced. At acetate concentrations below 500 mg l−1diffusion limitation partly masked the influence of sodium on the specific activity of the granular sludge. No adaptation of Methanothrix sp. to high sodium concentrations could be obtained in a period of 12 weeks. Net growth of Methanothrix sp. could be obtained at sodium concentrations of approx. 10 g l−1. At equal sodium concentrations, sodium chloride had a somewhat stronger effect on the specific activity than sodium sulfate, but the difference is unimportant for design purposes. The results obtained with short term activity measurements can reliably be used for predictions of the effect of sodium salts on continuously fed UASB reactors.