The ring-chaintautomerism of 5-hydroxy-2-isoxazolines has been investigated. By increasing bulk and/or number of substituents, the ring tautomer is generally favoured due to conformational factors in the chain tautomer. Introduction of two methyl groups at position 4 produces an exaltation of this effect (gem-dimethyl effect). The 5-substituent has also a steric and electronic influence on the reactivity