In this work, the long-term degradation of polymer pellets (also known as the primary microplastics/MPs) was monitored under laboratory-controlled conditions. Polymers of different origin such as the ABS, HDPE and PA (non-processed, recycled) were exposed to demineralized and artificial seawater, stored in room (20-24 °C) and cold (4-8 °C) temperatures during 20 months period. Surface topography and thermal characteristics of selected primary MPs were examined with the use of digital microscope and thermogravimetric (TGA) analysis. Release of secondary MPs into the water solutions was monitored with digital microscopy observations and µ-FTIR analyses. Additionally, leaching of the additives/plasticizers was studied by the GC-MS technique. Periodical, microscopic observations of the surface topography of single pellets show that some polymers undergo deformation over time in the water environment. According to results of the surface roughness, the external structure of the primary MPs (e.g. HDPE and PA) deteriorate after 6 months of exposure to water and cold temperatures. As indicated by the TGA analysis the water penetrates the polymer-matrix (HDPE), changing its physicochemical properties. The µFTIR analyses confirm that primary MPs fragment into smaller particles (secondary MPs), while exposed to various degradation factors (e.g. temperature, pH and salinity changes). The average size of all identified secondary MPs ranges between 13 and 1055 µm. The MP-like particles in the size range of ∼3-230 µm have been identified by the microscopic analyses. While exposing the primary MPs to different degradation factors for 20 months, no significant concentrations of phthalates were released into the water solutions.