Experimental studies have shown that dense C/C composites feature a complex surface ablation behavior under oxidation conditions. Their effective reactivity is not an additive property with respect to the intrinsic reactivities of their components. Typical surface roughness patterns progressively appear and the effective reactivity increases with time until it reaches a steady state value. In a previous work, the steady-state behavior of a 3D C/C composite has been modeled and explained. Here, the work has been extended to address the transient regime behavior. The surface roughness evolution and the effective reactivity are computed versus time. The model is validated by comparison with experimental data. These theoretical results provide for the first time a comprehensive understanding of the complex ablation process occurring at the surface of dense C/C composites. (C) 2017 Elsevier Ltd. All rights reserved.