The effect of the ionic environment on the adsorption of phenol from aqueous solutions was investigated in a predominantly microporous carbon and in a commercial carbon designed for wastewater treatment. It was found that not only the pH of the solution but also the method of its setting affects the adsorption capacity. Setting the pH with a buffer solution instead of HCl/NaOH results in a reduced adsorption capacity, owing to the increased number of competing species for adsorption sites, and also to pore blocking. The latter is less critical for the commercial carbon with wider pores. Thermal desorption of phenol exhibits an even stronger dependence on pH setting than adsorption. Upon heating, a mass equivalent to 10-35% of the adsorbed phenol is retained by the surface as a carbon-rich residue, which may modify not only the chemistry but also the pore volume and the pore size distribution of the carbon.