The micro-Raman spectroscopy was used for measuring the photo-polymerisation of composite resins. The elevated spatial resolution of the technique revealed surface non-homogeneities via the intensity variations of a strong band at 1400 cm(-1) and the presence of weak bands around the C-C and C=O stretching frequencies. Values of the degree of monomer conversion (DC) were determined for samples of Z100 commercial composite irradiated with different wavelength laser beams. Two methods were used for obtaining DC values. The first one exploits band decompositions in the spectral region of interest around 1608 and 1637 cut The non-uniqueness of the decomposition introduces relevant uncertainties. which are amplified by unwanted weak bands. An alternative method, avoiding any fit procedure and exploiting the band shape conservation under irradiation, was found producing more reliable results. The 632 nm line of the He-Ne laser was used as an exciting source. The measurements of the DC versus time are reported for three different wavelengths of the irradiation light. Preliminary, studies of the C=O stretching bands are also reported. Their intensities decrease under irradiation and there are indications of a behaviour similar to that of the C=C aliphatic band. (C) 2004 Elsevier Ltd. All rights reserved.