The structures and the photoinduced volume changes of obliquely deposited amorphous selenium were studied using Molecular Dynamics computer simulation. Obliquely deposited a‐Se thin films contain larger voids than the flatly deposited ones. Our computer simulations confirmed that the preparation method strongly determines the photoinduced changes of the chalcogenide glasses. We described in detail for the first time the complex atomic rearrangements occurring on photo‐excitation in porous samples, which are reversible and irreversible photo contraction/expansion, bond breakings and formations (both inter‐ and intrachain). A model for the kinetics of the photoinduced volume changes of flatly and obliquely deposited chalcogenide thin films is presented. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)