Biosorption isotherms define the relationship between the biosorption capacity of the biosorbent and the equilibrium concentration of the ions in solution, at a constant temperature. Experiments are routinely performed under near-equilibrium because it is impossible to determine the exact time at which equilibrium was attained. A novel attempt to study multi-ion biosorption in non-equilibrium conditions has been made, based on the Probability Isotherm theory. Materials and Methods: Probability Isotherm theory was examined with cucumber and kiwifruit peel beads which are reported to be efficient biosorbents. Te peels were incubated in a cocktail of seven ions (As, Cd, Cr, Cu, Hg, Pb, and Ni) at the same initial concentration (0.1-15 mg-1) and four different temperatures (25-55°C). Non-equilibrium biosorption data were modeled by the Langmuir isotherm model. Data were analyzed using a one-way ANOVA coupled with a Bonferroni posthoc test on GraphPad Prism 8 software. Cd and Ni ions showed the most well-defined trends with the Langmuir isotherm model. The binding of ions was Physico-chemical with simultaneously occurring physisorption and chemisorption reactions. Conclusions: Probability Isotherm theory can be applied to multi-ion biosorption in non-equilibrium conditions. The behavior of each ion is unique and no two biosorption systems are alike.