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We have studied the relaxation of conformers and the formation/relaxation of isomeric, weakly bonded dimers in pulsed supersonic expansions of seeded inert gases (He, Ne, Ar, Kr). The relaxation was determined from the intensity of a rotational transition for the higher energy species as a function of carrier gas composition, using the Balle/Flygare Fourier transform microwave spectrometer. Of thirteen molecules with rotational conformers which we examined, those with barriers to internal rotation greater than 400 cm−1 did not relax significantly in any of the carriers. The higher energy forms of ethyl formate, ethanol, and isopropanol, with smaller barriers, were not relaxed by He; those of ethanol and isopropanol were somewhat relaxed by Ne; and all were completely relaxed by as little as 5 to 20 mole percent of Ar or Kr in He or Ne. The relaxation in He or Ne is first order in the concentration of added Ne, Ar, or Kr as well as in the concentration of the high energy conformer. The pseudo first-order rate constants (larger in Ne than in He) increase sharply with Z of the rare gas, roughly in a 0:1:2:4 progression for He, Ne, Ar, and Kr, suggesting that the relaxation involves relatively long-range polarization effects. Similar behavior was found in the formation/relaxation of the weakly bonded dimer pairs: linear OCO–HCN, T-shaped HCN–CO2; linear FH–NNO and bent NNO–HF; and bent HF–DF and DF–HF. The case of the HCN/CO2 dimers is particularly striking. The T-shaped dimer was found first, using Ar as the carrier gas. Five years later the linear form was found with first run neon as carrier, but it could not be detected at all with Ar as the carrier. These results show that in favorable cases high energy species can be studied in supersonic expansions by freezing out a ‘‘high-temperature’’ concentration with a nonrelaxing carrier gas.