This work is part of a European research program aiming at the detection and quantification by ToF-SIMS of additives at polymer surfaces. An antioxidant has been mixed with different amorphous polymer solutions, and layers of different thickness (<1 pm) have been prepared by spin coating on silicon wafer. These samples were studied by ToF-SIMS. At constant additive concentration in solution (0.5 or 1 wt.%), SIMS peak intensities were found to depend on the film thickness. Data processing based either on univariate analyses or on multivariate analyses (principal component analysis) indicated a pronounced decrease of additive peak intensities below a critical film thickness (in the range of 10-80 nm). Within the range of studied samples, this transition did not seem to depend on the additive nature and/or concentration. However, the weight average polymer molecular weight (M,) was seen to play a key role. This was pointed out during the analyses of a polystyrene series with M, from 8300 to 63 000 g/mol and with a constant additive concentration (1 wt.%). These results indicate that a correlation may be found between the polymer chain dimension and the critical film thickness. The change of the macrochain conformation, from a mainly surface perpendicular orientation towards a more parallel orientation below the critical thickness, is evoked to explain these results. (C) 2004 Elsevier B.V. All rights reserved.