The first spectroscopic identification and characterization of ytterbium monoacetylide (YbCCH) is reported, By combining resonance-enhanced two photon ionization (R2PI), laser-induced fluorescence (LIF), and photoionization efficiency spectroscopy (PIE) with density functional calculations the (X) over tilde(2) Sigma(+) and the (A) over tilde (2) Pi(1/2,3/2) states of YbCCH as well as the (X) over tilde (1) Sigma(+) state of YbCCH+ have been characterized, The (A) over tilde (2) Pi(1/2,3/2) (X) over tilde (2) Sigma(+) system whose 0-0 band for the (A) over tilde (2) Pi(1/2) component lies at around 16 848 cm(-1) for YbCCH has been studied at 0.3 cm(-1) resolution, The excitation spectra, both R2PI and LIF are characterized by progressions involving the YbCC bending mode (upsilon(5)) whose wave number has been determined to be 96 and 103 cm(-1) for the (X) over tilde (2) Sigma(+) and (A) over tilde (2) Pi(1/2,3/2) state, respectively. The dispersed fluorescence spectra show a progression in the Yb-C stretching vibration with a wave number of omega(upsilon(3)) = 328 cm(-1). Density functional calculations confirmed the vibrational assignment and yielded a linear geometry for both the (X) over tilde and (A) over tilde state of YbCCH as well as for the (X) over tilde (1) Sigma(+) state of the cation. Photoionization efficiency spectroscopy yielded an adiabatic ionization potential of 47 165(10) cm(-1) [5.8477(12) eV]. Rydberg series converging to the 5(1) and 5(2) level of YbCCH+ were observed and combined with the appearance potentials led to omega(upsilon(5)) = 97 cm(-1) for the YbCC bending mode of YbCCH+. (C) 1997 American Institute of Physics.