We present an analysis of high-resolution spectra of three core-helium-burning ‘clump’ stars and two first ascent giants in the open cluster IC 4651. Atmospheric parameters (Teff, log g, vt and [Fe/H]) were determined in our previous study by Carretta et al. In this study, we present abundances of C, N, O and up to 24 other chemical elements. Abundances of carbon were derived using the C2 Swan (0, 1) band head at 5635.5 Å. The wavelength interval 7980–8130 Å, with strong CN features, was analysed in order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen abundances were determined from the [O i] line at 6300 Å. Compared with the Sun and other dwarf stars of the Galactic disc, mean abundances in the investigated clump stars suggest that carbon is depleted by about 0.3 dex, nitrogen is overabundant by about 0.2 dex and oxygen is close to solar. This has the effect of lowering the mean C/N ratio to 1.36 ± 0.11. The mean 12C/13C ratios are lowered to 16 ± 2. Other investigated chemical elements have abundance ratios close to the solar ones.