The global use of plastic as a cheap and versatile base material has been growing exponentially. This lead to a likewise increase in plastic waste of which a substantial fraction enters the ocean, but the further fate of plastic in the marine realm is not well constrained. Pathways of plastic degradation (physicochemical and biological) in the marine environment are largely unresolved; yet, microbial plastic degradation is a potential plastic sink in the ocean. However, there is a lack of methods to determine this process, particular if the overall turnover is in the sub-percent range. We developed a novel method for quantifying kinetics of microbial plastic degradation that is based on tracing isotopically labelled polymers through microbial food web structures. We tested our method with a Rhodococcus Ruber strain (C-208), a known plastic degrader, as a model organism. We used granular polyethylene (PE) (granule size 30 µm) that was almost completely labelled with the stable isotope 13C (99%) as a sole carbon source. We monitored CO2 concentration and its stable carbon isotope composition during 35-day incubations at atmospheric oxygen concentrations and found an excess production of 13C-CO2. This result provides direct evidence for the microbially mediated mineralization of carbon that was ultimately derived from the polymer. After terminating the incubation, we measured dissolved inorganic carbon (DIC) concentrations allowing us to determine the total excess production of 13C in the CO2 and DIC phase, and thus the rate of plastic degradation. Of the 2000 μg PE added, up to 1% was degraded over a time course of 35 days with a rate of up to 1.9 μg month-1, providing a first characterization kinetics of R. Ruber mineralizing PE. The results show that isotopically labelled polymers can be used to quantify plastic degradation at rates that are undetectable for classic gravimetrical methods.