The application of polarized absorption spectroscopy as a powerful method in the characterization of modified zeolitic molecular sieve crystals can supply information on (i) the electronic states, (ii) the alignment of zeolite-hosted colouring guests, and (iii) the concentration of the colouring species at very low loadings. The following examples are reported for modifications brought about during the synthesis: the isomorphous framework substitution (Co I I in AlPO 4 -5), the incorporation of ions (Se 2 anions in cancrinite), and the encapsulation of metal complexes (cobalticinium, [Co I I I (η 5 -C 5 H 5 )2] + in nonasils). The adsorption of chromophores (p-nitrodimethylaniline in AlPO 4 -5) is shown as an important post-synthesis modification.