In the present work, coral like hierarchical magnesium oxide (MgO) incorporated fly ash (FA) composite (FAMgO) synthesised by sol-gel method was evaluated for the adsorption of Reactive Black 5 (RB5) azo dye from aqueous solution. Adsorbent characterization using FE-SEM, XRD and FT-IR ascertained the formation of the FAMgO composite. Batch mode sorption studies were carried out for RB5 azo dye removal as a function of pH, FAMgO dose, initial dye concentration and temperature. The compliance of Langmuir isotherm model (R-2 = 0.999) corroborated the homogeneous nature of sorption of RB5 dye onto FAMgO with the sorption capacity (Q(max)) of 48.78 mg g(-1). The regression coefficient value revealed the best fit of pseudo-second-order model with the Q(e) value of 29 mg g(-1). The thermodynamic parameters such as Delta G degrees, Delta H degrees and Delta S degrees suggested the spontaneous and endothermic nature of RB5 dye sorption onto FAMgO and suitable mechanism has been proposed. (C) 2018 Elsevier B.V. All rights reserved.