The nature of dimethylalane, [Me 2 AlH] n , in the vapor phase and in hydrocarbon solutions has been reinvestigated experimentally by gas electron diffraction (GED), IR and mass spectrometric, and vapor pressure depression measurements, and also by quantum chemical calculations. The predominant vapor species at low pressures and temperatures near 470 K is the dimer (n ) 2), but at temperatures near 330 K the vapor consists of a mixture of the dimer and a smaller proportion (ca. 30%) of the trimer (n ) 3). The results of the GED measurements have been analyzed, together with those of ab initio calculations, using the SARACEN method to determine the most reliable structures of the doubly hydrogen-bridged dimer, Me 2 Al(µ-H) 2 AlMe 2 , and singly hydrogen-bridged trimer, Me 2 Al(µ-H)Me 2 Al(µ-H