In this work, an optical characterization of the hydrogen-air laminar combustions is presented in order to investigate the onset of cellular instabilities and quantify their intensity. Different combustion experiments of hydrogen-air have been developed in a cylindrical constant volume combustion bomb with centered ignition and transparent windows. The development of the flame and the wrinkling of its surface are recorded with a Schlieren technique. A cellularity density index (cell#/cm(2)) is used to quantify the phenomenon. The pressure during the combustion is registered with a piezoelectric transducer and its signal is treated by means of a two-zone diagnosis model. A parametrical study has been made by varying the fuel/air equivalence ratio, initial pressure and initial temperature during the combustions. In agreement with other published works, the present results show that hydrogen-air flames are more cellular as the mixture is leaner, with increasing values of the cellularity density. Similar trends are obtained as the initial pressure or the initial temperature increases. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.