We have analysed the spectrum of Arcturus (K2 III) relatively to the Sun, using a differential technique employing empirical models for both stars. We derive an effective temperature of 4260 ± 50K and a surface gravity log g = +0.90 ±0.35; these in turn lead to a very low mass, in the range 0.1 to 0.6 M⊙. Elements are found to be underabundant by an average factor of 4 compared with the Sun. The abundance patterns in the two stars are significantly different, in keeping with the belief that Arcturus is a star of an older generation than the Sun. The carbon isotope ratio, which is as small as 5 or 6, shows that the atmospheric material of Arcturus has been processed through the CNO cycle, and theoretical arguments also indicate that Arcturus is somewhat evolved.