by means of a two-step procedure. Full-dimensional rovibrational calculations are first carried out to obtain numerically exact rovibrational energies for J=0-15 in both parities. Effective spectroscopic constants for the vibrational ground state, ν ν ν ν 1 , ν ν ν ν 2 , and ν ν ν ν 3 are determined by fitting the calculated rovibrational energies to appropriate spectroscopic Hamiltonians. Combining our vibration-rotation corrections with the available experimental ground-state rotational constants, we also derive the new estimate for the equilibrium structure of HCO + , r e (CH)=1.091981(7) Å Å Å Å and r e (CO)=1.105615(2) Å Å Å Å, and for the equilibrium structure of HOC + , r e (HO)=0.990482(7) Α Α Α Α and r e (CO)=1.154468(2) Α Α Α Α. Regarding the spectroscopic parameters, our estimates are in exelent agreement with available experimental results for both HCO + and HOC + : the agreement for the rotational constants B v is within 3 MHz, for the quartic centrifugal distortion constants D v within 1 kHz, and for the effective ℓ ℓ ℓ ℓ-doubling constants q v within 2 MHz. We thus expect that our results can provide useful assistance in analyzing expected observations of the rare forms of HCO + and HOC + that are not yet experimentally known. Minimum energy path V MEP and effective bending potential adi Vv 1, v 3 for the (0,0), (1,0) and (2,0) stretching states along the Jacobi angle Ɵ Ɵ Ɵ Ɵ. The curves are shifted to coincide at Ɵ Ɵ Ɵ Ɵ=0 0 for HCO + and at Ɵ Ɵ Ɵ Ɵ=180 0 for HOC + .