Abstract A theoretical study of the CaC3+ and CaC3H+ systems has been carried out. Predictions have been made for some of the molecular properties (geometries, dipole moments, and harmonic vibrational frequencies) which could help in their possible experimental detection. In addition, a topological analysis of the electron density and its associated Laplacian has also been carried out. The predicted global minimum for CaC3+ is a rhombic four-membered ring 2d(2A1). Both doublet states of the other cyclic structure 3, 2B2 and 2A2, lie quite close in energy (about 4.0 and 9.6 kcal/mol, respectively, higher in energy at the G2(P) level). The linear isomer lies about 14.6 kcal/mol above the global minimum, at the G2(P) level. In the case of CaC3H+ the predicted global minimum is a bicyclic structure 2s(1A1). There are two isomers that lie close in energy: a linear species, 1t(2Σ), and a four-membered ring, 3s(1A1); lie about 16.2 and 19.4 kcal/mol, respectively, at the G2(P) level, above the global minimum. Except in the case of linear isomer, protonation favours the singlet states over the triplet ones. Low ionization potential and high proton affinities are obtained for the most stable CaC3 isomers. Therefore, if present in the interstellar medium, CaC3 should be easily ionized and would react quite easily to give the protonated species.