Explosion of fuel mixtures is a well-known physical phenomenon, with great importance in many fields. We investigate the 3-branches explosion limits of a hydrogen-oxygen system. Unlike most studies to date which often deal with a part of the ignition limits only, we investigate this phenomenon from a unified approach, by examining the molecular fluctuations in concentration of the chain-carriers within the system. To do so, we use methods from the field of Statistical Thermodynamics. This work presents a Grand Canonical Ensemble model aimed at evaluating the microscopic and macroscopic properties of the system and their fluctuations. Additionally, we perform a numerical simulation using a chemical reaction set, to examine the effects of the fluctuations on the ignition properties, namely to see under which conditions the transition from slow to fast reaction occurs. Preliminary results of the fluctuations analysis, compared with the experimental data and numerical simulations, had shown that the HO2is the dominant chain-carrier in the upper and intermediate explosion limits, when fluctuations in its concentration are above a critical value.