To establish the effect of KOH on the Raman spectra, microplastics from the following treatments were compared: untreated, exposed to ultrapure water at room temperature and to 10% KOH at 60 °C for 48 h. Further spectra were obtained from rayon and acrylic fibres exposed to 10% KOH at 40 °C for 48 h. Particles were transferred from filters onto quartz slides. Raman spectroscopy was performed using a 785 nm Renishaw inVia with Leica DM 2500 M microscope, 50× magnification lens. Particles were manually selected. Extended spectra were obtained at 1–5% laser power, 10 s exposure time and five accumulations. WiRE 4.1 software was used to process the data. BioRad KnowItAll was used to identify Raman spectra by individual and multi-components as well as agreement of spectral peaks.