Equilibrium between Al atoms, trimethylaluminum (TMA), and an Al-TMA complex has been observed over the temperature range 341-396 K at 50 Torr total pressure. Rate constants for the forward reaction are reported from 296 to 396 K. A van't Hoff plot of the equilibrium constant versus 1/T yields DeltaH(0) and DeltaS(0) values for the complexation reaction as -19.6 (+/-1.7) kcal mol(-1) and -27.7 (+/-4.8) cal mol(-1) K-1, respectively. An MP2 calculation gives DeltaH(0) of complexation as -12.6 kcal mol(-1), A complex which contains an Al-Al bond semi-bridged by a methyl group is predicted by ab initio and DFT calculations. (C) 2002 Elsevier Science B.V. All rights reserved.