Absorption and fluorescence properties of several cap analogues, namely nucleosides, nucleoside 5'-monophosphates and P1,P3-dinucleoside triphosphates derived from 7-methylguanine, N2,7-dimethylguanine and N2,N2,7-trimethylguanine, have been studied. The data obtained include the absorption and fluorescence spectra of the cationic (N1-protonated) and zwitterionic (N1-deprotonated) species, the pKa values of the ground and excited states of the methylated base moiety and the effect of temperature and solvent composition (mixtures of water and 1,4-dioxane) on the fluorescence intensity. Furthermore, the fluorescence lifetimes of N2,N2,7-trimethylguanosine 5'-triphosphate and P1-guanosine(5')-P3-[N2,N2,7-trimethylguanosine(5')] triphosphate have been determined as a function of temperature. These data clearly indicate that dynamic quenching must be taken into account when the extent of the intramolecular stacking of the latter compound is estimated by fluorescence spectroscopy.