The development of alternative renewable energies has taken importance due to the growing concern of climate change owing to greenhouse gas emission. In addition, population growth and their socio-economic development require large amounts of energy, converting the biomass in one of the most viable option for a sustainable future. In this sense, the olive pomace is a suitable candidate due to olive oil industry is one of the most important economic activity in Spain, which produces large amount of seasonal waste. On the other hand, steam gasification is considered to be one of the most effective and efficient techniques for generating hydrogen from biomass and electric power. In this sense, the cogasification of coal, petcoke and biomass is one of the alternatives to solve problems associated with gasifying each of them separately. Biomass and coal gasification could achieve synergistic effects, combining some of the characteristics of each feedstock to improve the characteristics of the gasification process such as tar formation or char reactivity. Furthermore, the use of catalysts during gasification can enhance the quality of released gas products, promoting tars and hydrocarbons reactions. However, its use can increase the operational costs, thus, the good selection of catalysts must be considered. In this regard, natural catalysts are growing their interest, being less expensive and more environmental friendly than synthetic catalysts. In this sense, dolomite is one of the most used catalysts because it is an inexpensive, natural and abundant catalyst. On the basis of the above, the aim of this research was to study the effect of dolomite as catalyst on the steam co-gasification process of coal-petcoke and biomass mixtures by means of the thermogravimetric analysis coupled with mass spectrometric (TGA-MS). In addition, the synergistic effect and the morphology of the ashes resulting from gasification process were also analysed. The use of dolomite enhanced the performance of the co-gasification process. The reactivity of these samples was improved, accelerating the process. However, the dolomite addition of 10 wt. % was not enough to improve the H2 yield, obtaining a lower [H2/CO] ratio in the catalytic samples than the non-catalytic samples. Nevertheless, it was observed that higher light compounds were generated due to the cracking of pyrolytic liquid. In addition, the porosity of remaining residues was enhanced due to the presence of the dolomite. Therefore, it could be considered a precursor for the active carbon formation from biomass.