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This study aimed to investigate the removal of a reactive dye from aqueous solution by adsorption. Activated carbon prepared from olive stone, an agricultural solid by-product, was used as adsorbent. Different amounts of activating agent (ZnCl2) and adsorbent particle size were studied to optimise adsorbent surface area. The adsorption experiments were conducted at different process parameters such as adsorbent dose, temperature, equilibrium time and pH. The experimental results showed that at equilibrium time 120 min, optimum pH ranged between 3 and 4, and adsorbent dosage was 2.0 g 200 ml−1. While the kinetic data support pseudo-second order, a pseudo-first order model shows very poor fit. Adsorption isotherms were obtained at three different temperatures (288, 298 and 308 K). The fitness of adsorption data to the Langmuir and Freundlich isotherms was investigated. In addition, the thermodynamic parameters such as isosteric enthalpy of adsorption (ΔHads)y, isosteric entropy of adsorption (ΔSads)y and free energy of adsorption ΔG0ads were calculated. BET surface area measurements were made to reveal the adsorptive characteristics of the produced active carbon. The surface area of the activated carbon produced with 20% w/w ZnCl2 solution was 790.25 m2 g−1.