In this work, the on-line combination of Thermogravimetric Analysis (TGA) and Fourier Transform Infrared Spectrometry (FTIR) has been applied to study the evolution with time of the volatile products evolved in the thermal pyrolysis of nannochloropsis sp. in order to characterize the different decomposition steps of this microalgal. The microalgal cells were treated in order to separate the lipid fraction, by breaking the cells and extracting the fraction Soluble in hexane, and both fractions, i.e., the extract and the extraction residue, have been also analyzed by TGA/FTIR, The results obtained for the IR spectra of the gases evolved in the pyrolysis of the samples (microalgae, extract and extraction residue) confirm the extraction of Compounds present in the microalgae whose decomposition are the responsible of the intensity or this band being related to the lipids present in the microalgae, On the other hand, the results show a different behaviour related to the main absorption bands analysed. The peak related to the C-H absorption band is shifted to higher temperature compared with the peak obtained for the other different functional groups analysed (C=O, CO2 and C-O-C), showing that the lipid compounds were probably released in the last decomposition stages of the main decomposition step observed in the thermogravimetric analysis. (C) 2008 Elsevier B.V. All rights reserved.