As microplastic particles enter the aquatic environment natural organic matter (NOM) molecules attach to the plastic surface first. These attached organic molecules are termed ecocorona. In a second stage microorganisms may attach at the plastic surface which is covered to some extend with the ecocorona and form a biofilm. Previously, it has been demonstrated that biofilm evolution is strongly depending on the surface chemistry of plastic particles (Lorite et al., 2011). These findings imply that the chemical composition of the ecocorona may vary and it may be dependent on the polymer type, the weathering stage of the polymer surface and the NOM composition in the water. To evaluate early stage sorption processes of NOM on different plastic surfaces we incubated artificially UV-aged and pristine polymer particles (PS, PET, LDPE) and a reference material (glass) in sea water from the Pacific Ocean. The sea water was obtained from various positions in the Pacific to account for possibly different NOM composition. Afterwards the organic matter was desorbed from the plastic surfaces, analyzed by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and compared to each other. To account for measurement uncertainty, which may play a crucial role for data interpretation various control and blank samples were analyzed in parallel. Here we present our preliminary results for the comparison of different polymer samples. In brief, we observed differences in the chemical composition of the ecocorona between aged and non-aged particles and between different polymers with about 50% of shared molecular formulas. For all ecocorona samples the proportion of aliphatic substances was over 90%. Further data analysis will be done to identify differences between the adsorbed NOM on the analyzed particles and link them to the NOM composition of the corresponding sea water samples. [1] Lorite et al., J. Colloid Interface Sci., 359, 289-295, 2011.