Threshold energies for the proton transfer reaction (CH-)-H-4 + C2H2 --> C4H2 + C2H- and for competitive collision-induced dissociation of the proton-bound complex of diacetylide and acetaldehyde, [HC4 . . H . . CH2CHO](-), are measured using guided ion beam tandem mass spectrometry. From the bimolecular proton transfer threshold, the gas-phase acidity of diacetylene is Delta(acid)H(0)(C4H2) greater than or equal to 1500 +/- 8 kJ/mol as a lower limit. The competitive dissociation is modeled using RRKM theory, yielding Delta(acid)H(0)(C4H2) = 1508 +/- 12 kJ/mol. Using the literature electron affinity of C4H, the derived bond dissociation energy of diacetylene is D-0(HC4-H) = 539 +/- 12 kJ/mol.