Plastic debris distributed in and around tributaries affects material cycling and biological activities in the downstream aquatic environment such as shorelines, open oceans and lakes. The distribution and dynamics of plastic debris in riverine environments were influenced by population density, the waste management, and the tributary system. The plastic debris can be transformed during their travel due to physical impacts, solar radiation and/or interactions with other anthropogenic litters, affecting to their interaction with colloidal particles in the river system. Polystyrene foam (PSF) is one of major plastic debris in the urban river in the capital city, Ulaanbaatar, Mongolia. PSF easily fragmented into many units as microplastics (MPs), further scattered to downstream with water flow followed by transformation on its surface and interaction with other colloidal particles. The collected PSFs along the shoreline of the urban tributary were carefully prepared through the process of wet decomposition of natural organic substances followed by sonication and filtration using a membrane filter. Plastics captured onto the filter was analysed using a micro-FTIR to obtain spectra and to identify colloidal particles adhered onto PSFs. The carbonyl index (CI), which is a surface degradation index calculated by IR absorbances, showed wide range values, indicating that surface degradation through oxidation and solar radiation was respectively occurred on distributed PSFs. Number of captured MPs in the view of a digital microscope was reaching to over 100 items on a piece of PSF. Various types of MPs adhered onto PSFs. Although there was no clear relationship between CI and abundances and/or types of adhered MPs, complex of MPs with PSFs can be a common status during their transportation in the urban river. Plastic debris actively interacts with a variety of secondary sourced MPs and travels together through direct adhesion or mediated by colloidal substances in the riverine environment.