This study investigated the formation and size distribution of microscopic plastic particles during the degradation of different plastic materials. Particle number concentrations in the size range 30 nm–60 μm were measured by nanoparticle tracking analysis (NTA) and Coulter Counter techniques. Each of the plastics used exhibited a measureable increase in the release of particles into the surrounding solution, with polystyrene (PS) and polylactic acid (PLA) generating the highest particle concentrations. After 112 d, particle concentrations ranged from 2147 particles ml−1 in the control (C) to 92,465 particles ml−1 for PS in the 2–60 μm size class; 1.2 × 105 particles ml−1 (C) to 11.6 × 106 for PLA in the 0.6–18 μm size class; and 0.2 × 108 particles ml−1 (C) to 6.4 × 108 particles ml−1 for PS in the 30–2000 nm size class (84 d). A classification of samples based on principal component analysis showed a separation between the different plastic types, with PLA clustering individually in each of the three size classes. In addition, particle size distribution models were used to examine more closely the size distribution data generated by NTA. Overall, the results indicate that at the beginning of plastic weathering processes chain scission at the polymer surface causes many very small particles to be released into the surrounding solution and those concentrations may vary between plastic types. •Degradation of seven polymers in aquatic medium results in an increased formation of microscopic plastic particles.•Weathering of polystyrene and polylactic acid generated the highest number of particles, especially in the nanometer range.•Surface erosion of all seven materials produced particles from 30 nm up to 60 μm.•Particle concentration increased with decreasing particle diameter.•Degradation processes start with chain scission at the polymer surface releasing many very small particles.