We report the first spectroscopic observation of the high Rydberg states of HCO. Individual lines in a system of vibrationally autoionizing Rydberg series converging to the (010) state of HCO+ are rotationally labeled in a double-resonance excitation scheme that uses resolved levels in the (010) A’ vibronic component of the 3p pi (2) Pi Rydberg state as intermediates. Observed high-Rydberg structure extends from the adiabatic ionization threshold-which falls just below the principal quantum number of 12 in the vibrationally excited series-to the (010) vertical threshold. Elements of a single series extending from n = 12 to 50, for which the total angular momentumless spin can be assigned as N = 1, are extrapolated to obtain a vertical convergence limit with respect to the 3p pi(2) Pi (010) A’ N’ = 0 intermediate state of 20 296.9 +/- 0.3 cm(-1). Referring this transition energy to the ground state, and subtracting the precisely known fundamental bending frequency of the cation, establishes the adiabatic ionization potential corresponding to the transition from HCO (2)A’ (000) J " = 0, K " = 0 to HCO+ J(+) = 0 (1) Sigma(+) (000). The result is 65 735.9 +/- 0.5 cm(-1) or 8.150 22 +/- 0.000 06 eV.