Theoretical spectroscopic parameters are derived for all isotopologues of HCO + and HOC + involving H, D, 16 O, 17 O, 18 O, 12 C, and 13 C 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 1 , ν 2 , and 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) = 1.091 98 Å and r e (CO) = 1.105 62 Å, and for the equilibrium structure of HOC + , r e (HO) = 0.990 48 Å and r e (CO) = 1.154 47 Å. Regarding the spectroscopic parameters, our estimates are in excellent agreement with available experimental results for the isotopic variants of 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-doubling constants q v within 2 MHz. We thus expect that our results can provide useful assistance in analyzing expected observations of the rare isotopo-logues of HCO + and HOC + that are not yet experimentally known.