The pressure and temperature dependence of electron attachment equilibria to 1-(trimethylsiloxy)-1,3-butadiene and 1,1,4,4-tetramethyl-1,3-butadiene were studied in n-hexane. The measured volume changes were less than expected for a full glassy solvent shell around the butadiene anion moiety, suggesting the substituents on the butadiene exclude at least one solvent molecule from the shell. The substituted anions are also less stable than the butadiene anion.