The ionisation of a series of substituted fluorenes has been measured. An H_– acidity function scale has been constructed for 40–95 mole % dimethyl sulphoxide in water containing tetramethylammonium hydroxide. The pK_a values of the carbon acids range from 17·96 (2-nitrofluorene) to 23·41 (9-t-butylfluorene), relative to the value of 18·59 for 9-phenylfluorene. These values are critically evaluated with regard to other acidity measurements. The acidity scale compares closely with that established using aniline indicators.