The [C4H7O]+ ions arising from 2-pentanone (1), 3-methylbutanone (2), 2-methylbut-3-en-2-ol (3), pent-3-en-2-ol (4), and 3-methylbut-3-en-2-ol (5) by methyl loss were studied by collisional activation. A triple sector (E/B/E) mass spectrometer was used to distinguish between fragment ion structures produced in the source and in the first field free region. The collision induced dissociations confirm or demonstrate that: (i) in the source, the methyl originally on the position 1 in compounds 1–5 is lost to give, respectively, [n-C3H7CO]+ (a), [i-C3H7CO] + (b), [CH3COHCHCH2] + (c), [CH3CHCHCHOH]+ (d), and [CH2C(CH3)CHOH]+ (e), while the methyl loss from the side chain gives, competitively, ion c for the five precursors; (ii) in the first field free region molecular ions 1,3,4, and 5 produce an identical mixture of structures a, b, and c; the molecular ion 2 leads essentially to acylium ion b. The mechanisms and the thermochemistry of the isomerization path of 1–5 are discussed.