Safety is a high-priority topic for the chemical industry to minimize the frequency and severity of accidents while keeping the productivity and quality of the production. The processes that may undergo thermal runaways due to exothermic reactions are at the heart of the risks of accidents. The study of such highly reactive systems is essential to achieve a safe and productive operation of existing processes and to ensure inherently safe new designs. It is well known that the stationary analysis with the van Heerden criterion must be satisfied, however, this is not sufficient to ensure reactor stability. Only dynamic analysis can provide an accurate answer concerning the safe operation of the reactor. Most of the reported stability studies are carried out for relatively simple systems (pseudo-homogeneous models, simple reaction schemes). This work presents the dynamic thermal stability study of a refining process (hydrotreatment of light cycle oils) carried out with real gasoils at industrial operating conditions. A 1D dynamic model that accurately represents the reactive system (gas-liquid-solid) was developed and validated with experimental pilot plant data. This mathematical model was used to perform the thermal stability analysis of the dynamic system using a perturbation method. The effect of the variation of the heat transfer coefficient on the thermal stability is presented. The spectral analysis of the eigenvalues indicating the stable/unstable behavior of the reactive system was compared with dynamic simulations. An excellent agreement was found between the simulations and the stability analysis. The case of oscillating behavior is also described. The frequencies of oscillation determined by the stability analysis are compared to the frequencies calculated by Fourier transform applied to the simulated signal. The reactor behavior and the oscillations features are accurately predicted with this stability analysis method. (C) 2009 Elsevier Ltd. All rights reserved.