Gasification characteristics of different chars obtained from the pyrolysis process of three species of microalgae (Scenedesmus almeriensis, Nannochloropsis gaditana and Chlorella vulgaris) has been studied by thermogravimetric–mass spectrometric analysis. The gasification process of microalgae chars was greatly influenced by the catalytic activity of indigenous mineral matter in microalgae samples. The presence of metals in microalgae samples influenced both, the samples reactivity and the production of gases. The catalytic effect of metals was more evident at high conversion values. Three models (volumetric model (VM), shrinking core model (SCM) and random pore model (RPM)) were used to describe the gasification process. However, good correlation of these models was only obtained up to conversion values of 0.6–0.7. At higher values, these models failed, as they do not consider catalytic effects during the gasification process. In this regard, a semi-empirical model was proposed. This model correctly predicted the gasification behavior of microalgae samples.