Vibrationally resolved photoelectron spectroscopy is combined with ab initio calculations to investigate the structure and chemical bonding in AlC2- and AlCSi-. AlC2- was found to have a C-2v structure whereas AlCSi- was found to be almost linear, thus establishing pi-coordination of Al in AlC2- and sigma-coordination in AlCSi-. The adiabatic electron affinities of AlC2- and AlCSi were measured to be 2.65(3) and 2.50(6) eV, respectively. The calculated vertical (2.87 eV) and adiabatic (2.60 eV) electron detachment energies for AlC2- agree well with the 2.73(0.03) and 2.65(0.93) eV experimental values, respectively. The calculated (2.86 eV) and experimental (2.64 +/- 0.04 eV) vertical detachment energies for AlCSi- were also in good agreement. The calculated vibrational frequency for AlC2 and vertical detachment energies for other higher energy features in both AlC2- and AlCSi- were also in good agreement with the experimental measurements. The combined experimental and theoretical effort allows us to elucidate the structures of AlC2- and AlCSi- and the nature of their chemical bonding.