Vertical ionization energies, characteristic vibrational frequencies, 1H n.m.r. signals, and electronic transitions are reported for acetylenes X·[CC]n·X (n= 1, 2, 3; X = H, CMe3, CH2·SiMe3, SiMe3). Comparison with the corresponding data of ethylene and butadiene derivatives indicates stronger electron back-donation to silicone Si â†� Cπ which is interpreted as being due to interactions with both of the perpendicular acetylene π-components. The inductive polarization +l(SiR3) > +/(CR3) can be estimated from the substituent effect of CH2·SiR3 groups.