The electronic and structural properties of porphyrin derivatives, such as chlorophyll a and pheophytin a, in pristine form and interacting with molecules of CO2 and O-2 are investigated through ab initio calculations. Both chlorophyll a and pheophytin a can adsorb molecules of CO2 and O-2 with binding energies around 0.2 eV indicating a physisorption regime. Modifications in the electronic properties of the resulting systems have been observed. These results are a consequence of the molecular levels of hybridization between the gases and the porphyrin molecules. Thus, these results open some possibilities to use those pigments as gas sensors. (C) 2010 Elsevier B.V. All rights reserved.