Long-range carbon atom topomerization in a 1,3-diyne has been demonstrated for the first time. 1-Phenyl-4-p-tolyl-1,3-butadiyne, C-13-enriched at C-1, was synthesized and subjected to flash vacuum pyrolysis. At 800 degreesC and 0.01 Torr, this resulted in nearly complete C-13 label equilibration between C-1 and C-2, as seen by NMR analysis. Pyrolysis at 900 degreesC further led to ca. 35% of the label migrating about equally to C-3 and C-4. These results demonstrate that both intrabond and interbond atom exchange processes are operative, with the former having a lower activation barrier. DFT and Moller-Plesset calculations support a mechanism that passes through Brown rearrangement (1,2-shift), closure to trialene (bicyclo[1.1.0]-1,3-butadiene), bond-shift isomerization to exchange C-2 and C-3, and ring opening, The resulting vinylidene can rearrange to a butadiyne with the isotopic label at C-3 or C-4. Consistent with earlier calculations, trialene is predicted to have alternating peripheral bonds, with a weak central a bond and significant diradical character. Trialene is predicted [(B3LYP/6-311+G(2d,p)] to lie 64.6 kcal/mol above butadiyne, with barriers of 2.2 and 4.4 kcal/mol, respectively, for ring opening or bond-shift isomerization. Other potential rearrangement mechanisms which pass through tetrahedrene (E-rel = 167.2 kcal/mol) or 1,2,3-cyclobutatriene (E-rel = 161.1 kcal/mol) lie at much higher energies.