Microfibres released from textiles are one of the most common types of microplastics (MPs) found in the environment. Whether they are synthetic or natural, they can undergo degradation in different environmental matrices. This may result in the leaching of a variety of chemicals, mainly textile dyes and additives of high toxicity that need to be regulated. A novel method was developed, validated and applied to identify and quantify these types of compounds present in a hydrolytic alkaline degradation solution (neutralized), but it can also be used in similar laboratory-simulated solutions, and seawater. The employed solution was utilized in an accelerated degradation simulation of two different polyester (PES) fibre types. Thirteen compounds were extracted and quantified using a solid-phase extraction protocol followed by HPLC-HRMS/MS. Intra-day (Intra-R) and inter-day (Inter-R) precision ranged from 0.02 to 6.23 % and 0.08 to 8.85 %, respectively, while linearity (R2) values were >0.9980. The limits of detection (LOD=0.7- 3.3 ng mL-1) and quantification (LOQ=0.5- 10 ng mL-1) were determined for the proposed method. Good recoveries were obtained for all compounds studied (65-120 %), while matrix effects ranged from -6 to 30 %, depending on the analyte. Ten compounds were detected and quantified in the degradation solution of the two different polyester fibres, with three (benzothiazole, 4-nitrophenol, 2,6-dichloro-4-nitroaniline) being PES type specific, while the rest were found in both types. A non-target analysis allowed the identification of a wider range of possible leachates (55 compounds in positive ion mode and 24 in negative).