Microplastics Monocorophium insidiosum, are frequently and widely used for toxicological assays. Because of their ecological importance, abundance, and sensitivity to various toxicants and pollutants, amphipods have long been known as sensitive environmental indicators. Amphipods are considered potential bioindicators in many environments. This research aims to quantify and simultaneously identify SMPs (5-100 µm), additives, plasticizers, and other components of microlitter ingested by organisms of Monocorophium insidiosum collected in the north basin of the Venice Lagoon in late fall 2018 and spring 2019. Organisms were pseudodigested without further denaturation/degradation of the particles and analyzed via Micro-FTIR Polymers in a wide range of densities were identified in all the samples. Some of the additives and plasticizers are related to specific polymers. Hence, they can be good proxies of the presence of SMPs. Besides, non-plastic synthetic fibers, such as rayon, and natural fibers, i.e., cellulose, were identified. Statistically significant differences were observed among the various samplings in abundance and particle composition.