Understanding the degradation of marine plastics is critical for assessing their environmental impact. This process requires standardization to enable interoperability and comparability of data. The goal of this study was to evaluate how spectral preprocessing, particularly baseline correction, influences the comparability of carbonyl index (CI) values across laboratories analyzing marine micro- and mesoplastics. Among the tested methods, the fillpeaks baseline correction provided the highest consistency and inter-laboratory correlation (0.82), supporting its use as a standard for environmental CI determination. Our results demonstrate that CI values derived from differently processed spectra are not directly comparable, highlighting the need for harmonized analytical protocols.