The inter- and intramolecular vibrations in the S-0 and S-1 state of catechol, d2-catechol, catechol(H2O)(1), and d2-catechol (D2O)(1) have been investigated experimentally by resonant two photon ionization (R2PI), spectral hole burning (SHB), and dispersed fluorescence spectroscopy (DF). The experimental frequencies are compared to the vibrational frequencies obtained from ab initio normal mode calculations using the 6-31G(d,p) basis set. In order to get a complete interpretation of the S-0 state spectra of d2-catechol the strong coupling of the two OD torsional motions has been taken into account. A two-dimensional calculation of the torsional eigenvalues based on an ab initio potential [-31G(d,p) basis] obtained from single point calculations is presented. Due to these calculations all vibrations in the S-0 state can be assigned. Furthermore a new assignment of the vibrations in the S-1 state of d2-catechol is given. In the case of catechol (H2O)(1) [d2-catechol(D2O)(1)] different structural isomers are discussed. Using HF ab initio calculations (including MP2, BSSE, and ZPE corrections) a trans-linear hydrogen bonding arrangement turns out to be more stable by an amount of 840 cm(-1) compared to a cyclic structure which is also a minimum of the PES. Normal mode calculations have been carried out for both structures and anharmonic corrections are calculated for the tau and beta(2) mode of the trans-linear arrangement. The prediction of the ab initio calculations is supported by the vibrational transitions observed in the spectra of the S-0 and S-1 state, which can be assigned on the basis of the vibrations calculated for the trans-linear structure. The most important feature of the R2PI spectrum of catechol(H2O)(1) [d2-catechol(D2O)(1)] is the occurrence of intermolecular vibrations of very low frequencies (14, 37 cm(-1)). These vibrations and the low frequency torsional modes in the spectra of the S-1 state of the catechol monomer strongly support the assumption that catechol is nonplanar in the S-1 state with respect to the OH groups. Due to this nonplanarity a double minimum potential for the intermolecular rho(1) mode of catechol(H2O)(1) is postulated. Using this assumption the low frequency vibrations of the R2PI spectra as well as the vibrations observed in the spectra of the S-0 state can be assigned.