We have studied the absorption bands around 14 μm in the spectra of 11 carbon stars with mass-loss rates ranging from 10−8 to 10−4 M⊙ yr−1, based on data obtained with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). All stars clearly show a C2H2 absorption band peaking at 13.7 μm, while the contribution from HCN molecules is small in this wavelength region. A simple plane-parallel LTE model with two layers at different temperatures is used to derive the C2H2 abundances in the outer photosphere and in the circumstellar envelope. We find that (1) the column density of the hot-layer, placed at about 3R * with a temperature of 1400 K is roughly the same for all stars regardless of the mass-loss rate, and (2) the contribution of cool molecules in the circumstellar envelope increases with the dust mass-loss rate, (3) the abundance of C2H2 in the two layers is about the same, i.e. no obvious depletion of C2H2 molecules seems to occur in the circumstellar envelope.