Pine bark was able to sorb cadmium, copper, and nickel ions from aqueous solutions. Binary equilibrium data from the combination of these metals were collected in this work using this sorbent. These data were modeled using three types of binary component equilibrium isotherms, all of which resulted in good fitting of the experimental data, with the Langmuir—Freundlich model resulting in their best representa- tion. In general, the capacity of bark for each metal in the binary system was lower than in the single metal systems. The study also examined the mechanisms of metal biosorption by bark. Scanning electron microscopy (SEM) and energy-dispersive x-ray (EDX) microanalyses revealed that metal ions were sorbed mainly at the cell wall of the bark and only a small amount of ions diffused into the cytoplasm. Both the EDX analysis and the atomic absorption spectrophotometry (AAS) measurements showed that ion exchange was an important mechanism in this sorption process. Electron spin resonance (ESR) tests demonstrated that free radicals from the sorbent also have a significant role in the sorption processes.