The formation of neutral [C 2 ,H 2 ,O 2 ] has been investigated by tandem mass spectrometry in a sector instrument and by energy-resolved collision-induced dissociation in a flowing afterglow-triple quadrupole apparatus. The neutral species are generated by two different methods: (i) neutralization of the distonic anion radical CH 2 COO by collisional electron detachment and (ii) collision-induced loss of halides X - concomitant with formation of [C 2 ,H 2 ,O 2 ] from α-haloacetate ions XCH 2 COO - . The tandem mass spectrometry results suggest that neutralization of CH 2 COO - and high-energy collisional activation of α-haloacetate ions lead to a mixture of α-acetolactone, c-(CH 2 C(O)O), and the acetoxyl diradical, CH 2 COO . Low-energy collisions with α-chloroacetate ions in the triple quadrupole analyzer produce α-acetolactone exclusively at the dissociation threshold. From the dissociation threshold measured for the appearance of Cl - from ClCH 2 COO - the heat of formation of acetolactone is determined to be ΔH f , 2 9 8 = -47.3 ± 4.7 kcal/mol.