Carbon cluster anions have been formed by a combination laser ablation/plasma generation method and then deposited in argon matrices and studied via Fourier transform infrared spectroscopy. In this paper a full isotopic study of the C-3(-) anionic cluster is reported. All six isotopomeric bands of the nu(3) antisymmetric stretching mode at 1721.8 cm(-1) (all C-12 isotopomer) have been observed and used together with a normal coordinate calculation to deduce that C-3(-) in Ar matrices is linear. This is in agreement with previous high-level ab initio calculations and with new density functional theory (DFT) and ab initio (MP2) results. Frequency shifts for all six C-3(-) isotopomers calculated by DFT and MP2 approaches match the observed shifts very well. Finally, evidence is presented that the C-3(-) anionic clusters are formed by electron capture and not by fragment aggregation.