Treatment of various diethynylgermanes, (R1)2Ge(C≡C–R2)2 (R1 = C6H5, CH3; R1–R1 = C6H4–C6H4; R2 = CH3, CMe3, nBu, C6H5), with equimolar quantities of di(tert‐butyl)aluminium or ‐gallium hydride, R2E–H (E = Al, Ga), afforded mixed alkenyl‐alkynylgermanes, (R1)2Ge(C≡C–R2)[C{E(CMe3)2}=C(H)–R2], by reduction of one of their C≡C triple bonds. The alkenyl groups have a cis arrangement of H and Al or Ga atoms across the C=C double bonds, which reflects the kinetically favoured situation. The cis/trans isomerisation is probably prevented from yielding thermodynamically favoured trans isomers; this is a result of an intramolecular bonding interaction between the α‐carbon atoms of the remaining ethynyl groups and the coordinatively unsaturated metal atoms. The higher Lewis acidity of Al compared to Ga results in relatively short Al···C(ethynyl) contacts (>231.2 pm) and a concomitant lengthening of the Ge–C bonds.