<p>Removal of Co(II) ions from aqueous solutions using chitosan-vanillin (polymer I) and chitosan-ortho-vanillin (polymer II) adsorbents has been investigated under different experimental conditions. The effect of pH, contact time, adsorbent mass, temperature, and initial concentration of studied ion on the sorption process has been studied using batch experiments. The quantity of residual ion has been estimated via atomic absorption spectrometry. The highest removal of Co(II) ions has been achieved at pH 4 with a maximum removal of 93.2% of ions after two hours of agitation. Langmuir isotherm model represents the best fit for the experimental data; whereas the highest chelation capacity was 5.899 mg g-1 and 7.651 mg g-1 for polymers I and II, respectively. Results from this study imply that chemisorption is the rate-determining step and thermodynamic parameters indicate that the uptake process is spontaneous and endothermic in nature.</p>