The 14 and 70 eV electron ionization mass spectra of five sets (R1 = Me, Et, i-Pr, t-Bu and Ph) of seven 2-aryl-4-R1-substituted (Ar = C6H4X; X = p-NO2, m-Br, p-Cl, H, p-Me, p-OMe and p-NMe2) (1-5) and of seven 2-aryl-5-phenyl-substituted 1,3-oxazolidines (6; for Ar, see above) were recorded to study their ring-chain equilibria in the gas phase. These equilibria were also studied by 1H NMR spectroscopy in CDCl3 for compounds 5 and 6. A few 2,4- and 2,5-dimethyl-2-aryl derivatives (7, 8: Ar = C6H4X; X = m-Br, H and p-OMe) were studied both in CDCl3 and in the gas phase. The main characteristics of the ring-chain equilibria expressed by the variable SigmaRA% of the ring and of the chain form proved to be a strong dependence on the nature of the substituents on C-2 and C-4. The results in the gas phase are compared with those in CDCl3.