Pyrolysis characteristics of three species of microalgae (SC (Scenedesmus almeriensis), NG (Nannochloropsis gaditana) and CV (Chlorella vulgaris)) have been studied by TGA–MS (thermogravimetric analysis coupled with mass spectrometry). The thermal behavior of microalgae samples could be described according to their biochemical composition and, in different extension, to their content in inorganic species. The high potassium content of sample SC led to the formation of a more stable char and the release of higher amount of volatiles. Pyrolysis kinetics were studied using a multiple-step model that successfully predicted the experimental behavior of these samples and was statistically validated. The gaseous products released in the pyrolysis of microalgae samples could be divided into light volatiles as H2, CO, H2O, CO2, light hydrocarbons and a condensable fraction formed by ketones, alcohols and aromatic compounds. Besides, nitrogen and sulfur compounds were generated in the form of amines, cyanides and hydrogen sulfides. Finally, an equation for predicting gas yields at a higher scale has been proposed. •Pyrolysis process of marine biomass was studied by TGA–MS (thermogravimetric analysis coupled with mass spectrometry).•High K content of microalgae samples led to more stable chars.•The kinetic analysis of the process was performed and the process was modeled.•H2 was produced at temperatures above 400 °C due to secondary reactions.•High molecular weight compounds (ketones, alcohols and aromatic) were detected.