La atom reactions with 1-butyne and 2-butyne are carried out in a laser-vaporization molecular beam source. Both reactions yield the same La-hydrocarbon products from the dehydrogenation and carbon-carbon bond cleavage and coupling of the butynes. The dehydrogenated species La(CH) is characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectra of La(CH) produced from the two reactions exhibit two identical transitions, each consisting of a strong origin band and several vibrational intervals. The two transitions are assigned to the ionization of two isomers: La(η-CHCCCH) (Iso A) and La(η-CHCHCCH) (Iso B). The ground electronic states are A (C) for Iso A and A (C) for Iso B. The ionization of the doublet state of each isomer removes a La 6s-based electron and results in a A ion of Iso A and a A ion of Iso B. The formation of Iso A from 2-butyne and Iso B from 1-butyne involves the addition of La to the C≡C triple bond, the activation of two C(sp)-H bonds, and concerted elimination of a H molecule. The formation of Iso A from 1-butyne and Iso B from 2-butyne involves the isomerization of the two butynes to 1,2-butadiene in addition to the concerted H elimination.