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Methods that unambiguously prove microbial plastic degradation and allow for quantification of degradation rates are necessary to constrain the influence of microbial degradation on the marine plastic budget. We developed an assay based on stable isotope tracer techniques to determine microbial plastic mineralization rates in liquid medium on a lab scale. For the experiments, 13C-labeled polyethylene (13C-PE) particles (irradiated with UV-light to mimic exposure of floating plastic to sunlight) were incubated in liquid medium with Rhodococcus ruber as a model organism for proof of principle. The transfer of 13C from 13C-PE into the gaseous and dissolved CO 2 pools translated to microbially mediated mineralization rates of up to 1.2 % yr−1 of the added PE. After incubation, we also found highly 13C-enriched membrane fatty acids of R. ruber including compounds involved in cellular stress responses. We demonstrated that isotope tracer techniques are a valuable tool to detect and quantify microbial plastic degradation. [Display omitted] • Method development to quantify microbial plastic mineralization by tracing stable carbon isotopes • 13C from 13C-labeled polyethylene (PE) was traced into 13C-CO 2 to determine mineralization rates. • Low mineralization rates were resolved: ~1.2 % year−1 of initially added PE (22.2 mg L−1). • Assimilation of PE-derived 13C into membrane lipids was observed. • Ability to quantify microbial degradation can improve modeling of Marine Plastic Debris turnover. [ABSTRACT FROM AUTHOR]