A carbon adsorbent has been prepared from flax shive by treatment with sulfuric acid. Several factors have been considered in the preparation: reaction time, temperature, and the amount and concentration of sulfuric acid. The optimised conditions of preparation were based on the cation exchange capacity, yield and sorption performance for the metals cadmium and mercury(II). These two metals behave differently when adsorbed on the carbon. Thus cadmium sorption follows the cation exchange capacity data and the optimum conditions for preparing the carbon for this and other metals sorbed by an ion exchange mechanism were: 12 moldm(-3) sulfuric acid at a ratio of 1:9 (weight flax shive:volume acid), 25-30 min reaction time at 200 degrees C. Similar preparative conditions were found to give the optimum results for mercury sorption except that in this case, the temperature was 160 degrees C. The sorbent shows a much higher uptake for mercury than cadmium, which suggests that the sorption mechanism for mercury sorption is not only ion exchange. The sorbents retained a high level of moisture (>80%), and elemental analysis shows very low amounts of sulfur (similar to 1%). Scanning electron microscopy shows that the carbon retains the fibrous texture of the original shive and X-ray diffraction shows that the carbons are amorphous. The presence of carboxyl, carbonyl and hydroxyl (or phenolic) groups is confirmed by infra-red spectroscopy.