The Âreverse charge parityÂ model proposed by us establishes the selective electrostatic binding of charged proteins with oppositely charged functional iron oxide nanoparticles (IONP) in an aqueous solution [Ghosh et al 2014Mater. Res. Express 1 015017]. In this paper, we have investigated the selectivity in binding of charged proteins with oppositely charged functional IONP in a binary protein solution. IONP was surface functionalized both positively (e.g., coated with cetylpyridiniumiodide, or CPI) aswell as negatively (e.g., coated with tri-lithiumcitrate, or TLC). The binary protein solutionwas prepared bymixing a 1:1weight ratio of hen eggwhite lysozyme (HEWL) and ovalbumin (OVA) inwater.HEWL(pI 11) is positively charged and OVA (pI 4.5) is negatively charged in water. The binding of proteins with functional IONPwas characterized using several techniques, like, circular dichroism(CD), ultraviolet-visible (UV-vis), and fluorescence spectroscopy, ïº-potential and DLS. The results confirm the application of Âreverse charge parityÂ model for selective binding of proteins with functional nanoparticles even in a mixed protein environment. The effect of counterions (e.g., I¯ and Li+) on the protein conformation has also been discussed briefly.