The aromaticity of three-membered monoheterocycles containing a group 14 or 4 element was investigated by using modern aromaticity criteria: structural, electronic (FBO), optical (vibrational spectroscopy), energetic (ISE), and magnetic (NICS-scan, GIMIC). The results of all methods used show that the group 14 compounds are aromatic, with cyclopropene exhibiting σ aromaticity, whereas sila-and germacyclopropenes X 2 EC 2 R 2 exhibit pseudo-π aromaticity due to the interaction of the CC π bond orbital with the low-lying vacant antibonding σ* orbital of the X 2 E fragment (E = Si, Ge). At the same time, the results of MO and magnetic descriptor calculations testify against the aromaticity of group 4 cyclopropenes Cp 2 MC 2 R 2 (M = Ti, Zr, Hf). Thus, a fundamental dissimilarity in bonding schemes and electronic structure was disclosed between 1-heterocyclopropenes containing E = C, Si, Ge atoms and those containing M = Ti, Zr, Hf atoms, the latter molecules being in essence π complexes of acetylene.