The ultraviolet photoelectron spectrum of HXC=C- (X=C2H3) is reported along with Qb initio calculations. The adiabatic electron affinity of the X(1)A’ state is measured to be 0.914(15) eV for C4H4 and 0.909(15) eV for C4D4. The term energy of the C4H4 a ($) over tilde(3) A’ state is 1.923(15) eV and the b (3)A " term energy is 2.035(30) eV. Geometries and frequencies of several stationary points on the C4H4 and C4D4 neutral and anion surfaces at the configuration interaction with singles and doubles level. of theory are reported, as well as an intrinsic reaction coordinate calculation at the restricted Hartree Fock level on the C4H4 Singlet surface. Calculations and experiment are combined to estimate the lifetime of singlet vinylvinylidene for rearrangement to vinylacetylene to be 20-200 fs, corresponding to lifetime broadening of 35-3 meV.