Self-limiting gas-surface reactions lead to reaction fronts that penetrate nanoporous materials with a finite speed. We present a closed form theoretical model, validated against molecular simulations, that shows the influence of the fractal scaling law on the time needed to fully penetrate fractal agglomerates of nanoparticles. For very large agglomerate sizes, this penetration time scales with the number of particles N in the agglomerate as NDf-1/Df. The penetration time for agglomerates with fractal dimensions D-f < 3 may therefore be orders of magnitude smaller than for non-fractal porous materials. (C) 2019 Elsevier Ltd. All rights reserved.