**Abstract**

 In the last 60 years, plastic has become a widely used material due to its versatility and wide range of applications. This characteristic, together with its persistence, makes plastic waste a growing environmental problem, particularly in the marine ecosystems. The production of plant-derived biodegradable plastic polymers is assuming increasing importance. Here, we report the results of a first preliminary characterization of carbon stable isotopes (δ 13C) of different plastic polymers (petroleum- and plant-derived) and a first experimental study aimed to determine carbon isotopic shift due to polymer degradation in an aquatic environment. The results showed that the δ 13C values determined in different packaging for food uses reflect the plant origin for "BIO" materials and the petroleum-derived source for plastic materials. Considering degradation, δ 13C values of both bio bags and HDPE bags showed a gradual decrease toward less negative values when kept immersed in seawater, recording a δ 13C variation (Δδ13C) of 1.15 and 1.78‰, respectively. With respect to other analytical methods, the characterization of the plastic polymer composition by isotope ratio mass spectrometry is advantageous due to low cost and rapidity of analysis, small amount of sample required, high sensitivity, and the possibility of analyzing colored samples.

**Keywords:** carbon isotopes, plastic polymers, EA/IRMS, plastic degradation, plastic pollution
