This thesis is comprised of studies in the characterisation of monolayer-protected and metal cluster of the structural response of size-selected (bare) clusters to chemical reactions. The technique employed is high-angle annular dark field (HAADF) aberration-corrected scanning transmission electron microscopy (AC-STEM). The effect of chemical reactions on size-selected metal clusters was investigated. The clusters under investigation were imaged with AC-STEM and their structure was assigned by comparing the atomic resolution images with a set of multi-slice STEM image simulation atlases. The effect of vapour-phase 1-pentyne hydrogenation on size-selected Aux (x=923 and 2057) cluster was studied and it was found that the gold nanoclusters demonstrate high stability in both size distribution and structure under the reaction. On the contrary, size-selected Pdx (x=923 and 2057) clusters tended to transform from amorphous to high symmetry structures under the same reaction condition. The gas-phase CO oxidation reaction on size-selected Aux (x=561, 923 and 2057) cluster was studied with regard to cluster size distribution and atomic structure. It was found that under the same conditions of the CO oxidation reaction, two different kind of ripening modes could be identified depended on the cluster size. Smoluchowski ripening, in which clusters diffuse intact and coalescence, is found to occur for Au2057 in the CO oxidation reaction. Ostwald ripening, in which larger clusters grow at the expense of smaller ones, was found to occur for Au561 and Au923 clusters, due to the extra energy generated from catalytic CO oxidation reaction.