© 2018 American Chemical Society. Aluminum carbide cluster anions AlnC2- (n = 5-13) were observed as the most dominant products in the gas-phase reactions of laser-ablated Aln- with organic molecules, such as methanol, ethanol, pentane, acetonitrile, or acetone. Density functional theory calculations predicted two possible isomeric structures for AlnC2-: isomers in which two carbons are dissociated (type D) as in the case of the bulk aluminum carbide and novel isomers in which two carbons form an acetylide-like C2 unit. The latter isomers are further categorized into three types depending on the location of the C2 unit: the C2 unit is encapsulated within the Al cage (type I), contained in the surface of Al clusters (type S), or attached to the surface of Al clusters (type O). Size-dependent behavior of the adiabatic electron affinities of AlnC2 determined by photoelectron spectroscopy was explained in terms of polymorphism as a function of size (n): type I for n = 5-8, type D for n = 9-11, type D or O for n = 12, and type O for n = 13. The tendency in which the position of the C2 unit was shifted from the inside to outside with the increase in n was ascribed to the balance between the stabilizations gained by forming Al-C bonds and Al-Al bonds. The smaller AlnC2- clusters (n = 5-8) prefer to surround the acetylide-like C2 unit with the Al atoms so as to maximize the number of Al-C bonds, whereas larger ones (n = 12 and 13) prefer to attach the C2 unit onto the surface of the Al clusters so as to maximize the number of Al-Al bonds.