On electron impact (EI) ionization, two &/trans pairs of 4methyl-5-phenyl and 4-phenyl-5-methyl regioisomeric karbethoxy-2-isoxazolines showed normal mass spectra and mass-analysed ion kinetic energy (MIKE) spectra of metastable (MI) and collision-activated (CA) molecule ions, allowing unequivocal differentiation of the regioisomers. The cisltrans stereoisomers of each regioisomer showed very similar normal mass spectra. Very interestingly, the cis-and trans-4-phenyl-lmethyl stereoisomers appeared reasonably differentiated by the molecule ion MIKE spectra, whereas the 4-methyl-5-phenyl regioisomeric pair of stereoisomers did not. The influence of the phenyl substituent to the fragmentation processes was notable. Some fragments of interest were studied by comparison of their MIKE spectra with those of model ions, generated by EI from suitable substrates, including (i) the isomeric a,&unsaturated oxime, namely ethyl (Z)-2-(hydroxymino)-3-methyl-4-phenylbut-3-enoate, a by-product of importance for the mechanism(s) of the addition/cycloaddition reactions of nitrile oxides to alkenes and (ii) trans-& methylstyrene, a dipolarophilic reactant in the same reactions. The favoured heterocyclic C(5)-O( 1) bond cleavage occurred only for the ionized 4-methyl-lphenyl 2-isoxazoline pair, leading to a distonic ion of relevance, as it can represent either a reasonable precursor for both the isomerization to the ionized a,&unsaturated oxime and the EI-induced cycloreversion yielding ionized &methylstyrene, or the ionized form of a zwitterionic intermediate, which had been proposed previously for the addition/cycloaddition mechanism(s) in the solution phase, currently under study. 2 5 6 3 7 8 Scheme 1.