It has previously been shown that when a gas is adiabatically expanded, association of the molecules occurs as a first step in the condensation process. It is now shown that double molecules exist in the gas even before expansion. These double molecules have been observed in the mass spectra of carbon dioxide, nitrous oxide, nitrogen, oxygen, argon and xenon, and some gas mixtures. They are believed to exist in all gases at temperatures where the attractive van der Waals interaction is important, and where the pressure is sufficiently high to produce a significant fraction of three-body collisions. Evidence is presented that the observed molecules are not the products of reactions in the mass spectrometer. For argon and xenon the concentrations of these species, and their variation with temperature over the range 180 to 300 $^\circ$K, agree reasonably well with the theoretically expected values. For polyatomic molecules the measured concentration is much less than predicted. This is believed to be associated with an instability of the double molecules, the transfer of rotational energy from the constituent molecules breaking the van der Waals bond. A theoretical assessment of this effect is attempted for carbon dioxide and nitrogen, and gives approximate agreement with experiment.