In this paper structures of resorcinol (1,3-dihydroxybenzene) and hydroquinone (1,4-dihydroxybenzene) cations are presented. The adiabatic ionization potentials of the dihydroxybenzenes and the vibrations of the ionic ground state of the dihydroxybenzene cations are experimentally obtained by mass analyzed threshold ionization (MATI) and infrared photoinduced Rydberg ionization (IR-PIRI) spectroscopy. In contrast to the catechol (1,2-dihydroxybenzene) cation, two isomers of different symmetry can be obtained for resorcinol and hydroquinone cations. The MATI and IR-PIRI spectra of the isomers of resorcinol will be interpreted by comparing the experimental results with the results of density functional theory (DFT) and ab initio calculations performed at the complete active space self-consistent field (CASSCF) level of theory. Furthermore, the adiabatic ionization potentials as well as the excitation energies (for the S-1<-- S-0 transition of neutral dihydroxybenzenes) are derived from CASSCF calculations for the S-0, S-1, and D-0 states including second order Moller-Plesset (MP2) and zero point energy corrections. The experimentally observed ionization potentials of all isomers of all dihydroxybenzenes can be predicted from the calculated ionization potentials with excellent accuracy. The relative shifts of ionization potentials of different isotopomers can also be predicted. In this paper partially deuterated d1-resorcinol up to d5-resorcinol is investigated. By comparing the experimentally observed excitation energies and ionization potentials (between deuterated and undeuterated resorcinol) with the calculated values, an assignment of the electronic origins of the resonant two-photon ionization (S-1<-- S-0 excitation) and MATI spectra to different isotopomers can be given.