Ab initio calculations have been carried out on the lowest energy isomers of C4H5 to predict spectroscopic properties and relative stabilities. Geometry optimizations were carried out on stationary point conformations of seven configurational isomers at UHF, B3LYP, MP2, CISD, QCISD, MCSCF(7,7), and MCSCF(9,9) levels of theory. Single-point energies were evaluated at the MP4, CCSD(T), MCSCF(11,11), and multireference CISD levels. Disparities as large as 70 kJ mol(-1) are found between relative energies predicted by single- and multireference methods for the same isomer. Comparison to experimental values suggests that the multireference methods inadequately model the relative correlation energy. Zero-point corrected relative energies (in kJ mol(-1)) obtained at the QCISD level with a 6-311 G(d,p) basis set are the following: 2-butyn-1-yl (0); 1-buryn-3-yl (10); 1,2-butadien-4-yl (13); cyclobuten-3-yl (17); 1,3-butadien-2-yl (50); I,3-butadien-1-yl (59); and 1-butyn-4-yl (64), Relative energies calculated by multireference methods are higher, and decrease slowly as the active space size increases. Relative energies and hyperfine constants obtained at the QCISD level are in agreement with, available experimental data and empirical estimates. Some of these isomers are candidates for relocalization, a phenomenon that results in predicted multiple minima and unusually flat vibrational potential energy surfaces for the isoelectronic C3H3O isomers. Of the present series of molecules, only 2-butyn-1-yl exhibits an especially fiat bending potential along the appropriate isomerization coordinate. Predicted vibrational transition frequencies and intensities, dipole moment components, Fermi contact hyperfine constants, and conformational potential energy curves are presented.