The overarching aim of this work was to assess the performance of low cost and readily available biomass, Yerba Mate (Y-Mate), for the removal of Orange II dye (OII) and Methylene Blue (MB) as a function of solution pH, dosage and particle size at different initial dye concentrations and contact time. The data was quantitatively described through appropriate isotherms and kinetic equations. OH biosorption on Y-Mate significantly decreased with increasing pH from 2 to 6, whereas, MB biosorption increased steadily with increasing pH. The Sips isotherm model best represented biosorption isotherms for both contaminants. Under the experimental conditions explored, the Langmuir maximum biosorption capacities reached 47 and 52 mg/g for OII and MB, respectively. The biosorption kinetics were well represented with the pseudo second-order equation. Surface area and point of zero charge (PZC), thermo-gravimetric analysis (TGA), scanning electron microscope (SEM), fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analytical techniques were considered to characterise the surface chemistry of Y-Mate. The analysis indicated that the mechanisms of Y-Mate towards both anionic and cationic dyes are mainly through the co-action of pi-pi. interaction and hydrogen bonding, while electrostatic attraction had an additional effect on the biosorption. (C) 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.