The risk assessment in terrestrial environments has been scarcely studied for mixtures of organic contaminants. To estimate toxicity due to these compounds, an ecotoxicological test may be done with the appropriate organism and biomarker. Photosynthesis is principally performed at photosystem II, and its efficiency is affected by any environmental stress. Consequently, the measure of this efficiency may be a good indicator of toxicity if different parameters are employed, e.g., the quantum efficiency of photosystem II and the photochemical quenching coefficient. We did a series of assays to determine the toxicity of two organic contaminants, ibuprofen and perfluorooctanoic acid, using a higher plant (Sorghum bicolor). The results showed more toxicity for the perfluorinated compound and greater sensibility for the quantum efficiency of photosystem II. Regarding the binary combination, three methods were applied to calculate EC50: combination index, concentration addition, and independent action. Synergistic behavior is the principal toxicological profile for this mix. Therefore, the combination index, which considers interactions among chemicals, gave the best estimation to determine risk indices. We conclude that the inhibition of photosynthesis efficiency can be a useful tool to determine the toxicity of the mixtures of organic pollutants and to estimate ecological risks in terrestrial environments.