Hi Jean-Francois, There is not yet a command in the standard Chimera distribution to change volume surface transparency. There is such a command in the optional package called "Animation Commands" on the experimental features web page: http://www.cgl.ucsf.edu/chimera/experimental/experimental.html http://www.cgl.ucsf.edu/chimera/experimental/transition/transition.html You can install this with your current Chimera and then use "transition transparency". Tom

Hi Jean-Francois, The "Animations Commands" plugin on the Chimera experimental features page adds the "transition" command. There is no new tool in the menus. The details link on the exper features web page explains its use http://www.cgl.ucsf.edu/chimera/experimental/transition/transition.html Make sure you get the version that is compatible with your Chimera version. See the "older versions" link on the exper features page if you are using Chimera older than version 1.2348 (Feb 2007). Tom