The mechanisms of the three reaction pathways for the photochemical transformation of 3,5-dimethylisoxazole (1) in its first singlet excited state (pi ->pi*)(1) have been determined using the CASSCF (11-orbital/14-electron active space) and MP2-CAS methods with the 6-311G(d) basis set. These three reaction pathways are denoted as (i) the internal cyclization-isomerization path (path A), (ii) the ring contraction-ring expansion path (path B), and (iii) the direct path (path C). This work provides the first theoretical examinations of mechanisms for such photochemical rearrangements. The present theoretical findings suggest that the photoisomerization of 1 via path C should be much more favorable then either path A or path B. Nevertheless, the theoretical observations reveal that path B, which consists of a sequence of small geometric rearrangements, should be energetically feasible as well. Accordingly, the fleeting intermediate, acetyl nitrile ylide (4), which arises from the mechanism of path B, can be detected experimentally.