The ability of green algal biomass of Ulva sp. to biosorb Cr(III) and Mn(II) from single- and binary- solute systems was investigated. Experimental results showed that pH 4.5 resulted in highest metal uptake by Ulva sp. biomass. Determined levels of maximum biosorption capacities were 2.89 and 1.07 mmol g-1 for Cr(III) and Mn(II), respectively, during single-solute systems. Isotherms were modeled using the Langmuir, Freundlich, and Redlich-Peterson models with the later able to describe the biosorption isotherms with high correlation coefficients and low % error values. The removal kinetics were described using pseudo -first and -second order models. Desorption was possible using 0.01 M HCl and with this elutant, Ulva sp. biomass was reused for three cycles of sorption-desorption. Experiments with binary metal solutions showed that Ulva sp. biomass preferred Cr(III) (2.03 mmol g -1) compared to Mn(II) (0.34 mmol g-1) at pH 4.5. Binary isotherms were described using the extended Langmuir, Freundlich, and Redlich-Peterson models incorporated with interaction factor. © 2013 American Institute of Chemical Engineers.