Porous silicon (PS) samples were prepared under different anodization conditions and correspondingly the sensor parameters were studied for organic vapours like methanol, ethanol, propanol, acetone and benzene at low ppm level using capacitive measurements. The surface morphology of the samples was characterized by scanning electron microscopy (SEM), whereas Raman spectroscopic studies were made to identify the surface species formed due to surface passivation. Selective response was observed for either methanol or ethanol depending on the combined effect of the pore morphology of PS, its surface passivation and the concentration dependent molecular kinetics in vapour phase. A probabilistic approach of the concentration dependent vapour molecular kinetics is demonstrated to explain the sensor performance and selectivity studies. © 2010 Elsevier B.V. All rights reserved.