Aldehydes are known to be present in PET preforms and PET bottles. However, the analysis of these aldehydes is difficult because of the high crystallinity of PET and because of its insolubility in common solvents. Titration of acetaldehyde was investigated comparatively by four different methods : by the solid/gas headspace technique (s/g HS), accepted industrially as the standard method, by colorimetry, based on complexation with pararosaniline dye, by colorimetry, based on derivatization as 2,4-dinitrophenyhydrazones, and by head-space liquid/gas chromatography (1/g HS). Similar results were achieved by the last three methods. In contrast, the standard solid/gas headspace chromatography yielded data that was one order of magnitude smaller. The presence of formaldehyde in PET preforms was detected by liquid/gas headspace chromatography only.