Naphthenic acids are recognized as the main corrosive species in acidic crude oils, although they represent less than 3 wt%. Alternative methods have been developed in an attempt to remove naphthenic acids from the oil, however, their implementation in petrochemical industry still represents a challenge. Herein, a sub-product of the steel industry, steel slag, is evaluated as an economic alternative catalyst and environmentally feasible to remove naphthenic acids present in crude oils. A high-acidity crude oil (TAN = 4.79 mg KOH g(-1) and S = 1.022 wt%) was submitted to thermo-catalytic process at 300 and 350 degrees C during 2, 4 and 6 h and its degradation products were monitored by negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron mass spectrometry (FT-ICR MS), total acid number (TAN) and total sulfur. The main crystalline phases detected by X-ray diffractometry in the catalyst were calcite (CaCO3), silica (SiO2) and magnesia (MgO). Among them, the MgO contributes effectively to promote the thermo-catalytic decarboxylation of naphthenic acid species, with a TAN reduction of 43.50% (4.79 -> 1.89 mg KOH g(-1)) from the original oil to the degraded oil obtained after treatment at 350 degrees C for 4 h. Acid species with lower pK(a) values were selectively removed with the ESI(-)FT-ICR MS data confirming an increase in the DBE values from 1-5 to 5-18 for O-2 class. Therefore, the catalyst selectively promoted the removal of naphthenic acids via (i) neutralization reaction; (ii) cracking reaction and (iii) MgCO3 formation from CO2 molecules produced by a previous thermal decarboxylation reaction. (C) 2015 Elsevier Ltd. All rights reserved.