We report an investigation into the effects of the vapour-phase hydrogenation of 1-pentyne on the atomic structures of size-selected Au and Pd nanoclusters supported on amorphous carbon films. We use aberration-corrected high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) to image populations of the nanoclusters at atomic resolution, both before and after the reaction, and we assign their atomic structures by comparison with multi-slice image simulations over a full range of cluster orientations. Gold nanoclusters consisting of 923 ± 20 and 2057 ± 45 atoms are found to be robust, exhibiting high structural stability. However, a significant portion of Pd923±26 nanoclusters that appear amorphous prior to treatment are found to exhibit high symmetry structures post-reaction, which is interpreted as the reduction of oxidised Pd nanoclusters under the reaction conditions.