Microplastics (MPs) are abundant in aquatic systems. The ecological risks of MPs may arise from their physical features, chemical properties, and/or their ability to concentrate and transport other contaminants, such as per- and polyfluoroalkyl substances (PFAS). PFAS have been extracted from MPs found in natural waters, but there is no mechanistic investigation of the effect of PFAS chemistry and water chemistry on how PFAS partition onto secondary MPs. Here, we studied the influence of pH, natural organic matter (NOM), ionic strength, and temperature on the adsorption of PFAS on MPs generated from PET water bottles. The adsorption of the PFAS to the MPs was spontaneous at 25C, based on Gibbs free energy (G -16 kJ/mol), primarily due to increased entropy. Adsorption reached equilibrium within 7-16 h. Hence, the PFAS will partition to the surface of PET MPs within a few hours to days in fresh and saline waters. Natural organic matter decreased the capacity of PET MPs for PFAS through electrosteric repulsion while higher ionic strength favored PFAS adsorption by decreasing electrostatic repulsion. Increased pH increased electrostatic repulsion, and negated PFAS adsorption. The study provides fundamental information for developing models to predict interactions between MPs and PFAS.