The study investigates heterogeneity of adsorption energy in Engelhard Titanium silicate, ETS-4. The adsorbent was synthesized with two different initial Si:Ti ratios and was characterized physically using Scanning Electron Microscope (SEM) and X-ray diffraction (XRD). Equilibrium isotherms were volumetrically measured for oxygen, nitrogen, and methane, and the isosteric heat of adsorption at different adsorbate loading was evaluated. In both adsorbents, the isosteric heat decreased with increased loading. The results seem to suggest that the distribution of adsorption energy in ETS-4 is related to the Si:Ti ratio. The energy distribution is broader for the smallest oxygen molecule, as expected. This study gives insight into energetic heterogeneity, its probable origins, and its effect on adsorption phenomena in ETS-4. (C) 2004 Elsevier Ltd. All rights reserved.