A methodology for the investigation of photocatalytic degradation processes of organic molecules involving surface-enhanced Raman spectroscopy (SERS) is developed and applied to the investigation of an azo dye. Initially, the development of substrates to enable SERS to be used for sensitive in situ characterization of photochemical processes in thin-films materials was focused. An optimized method for the preparation of a surface that is altogether photocatalytic (by the presence of semiconducting TiO2) and SERS-active by the presence of nanostructured silver) was developed. The optimized SERS-active substrate was then employed to study the photodegradation reaction of Congo red. Considering that photocatalytic degradation process of organic molecules adsorbed on TiO2 might involve either their oxidation or reduction reaction, the spectroelectrochemical study of the dye was also performed, in order to clarify the transformations involved in initial steps of its photochemical decomposition. The approach here described involves combination of data relative to the photochemical behavior of dye and its electrochemical study - using enhanced spectroscopic techniques for identification and characterization of products - and proved to be very promising, in the sense of offering relevant information about the complex mechanisms that are involved in photodegradation processes of organic molecules. (c) 2006 Elsevier B.V. All rights reserved.