The clusters discussed in this paper will be referred to using the following nomenclature assignment: (x, y, z ) = (NH 3 ) x (HIO 3 ) y (I 2 O 5 ) z H + . There are several patterns that seem to appear as the clusters begin to grow larger in size. One of which involves the (3, 2, z ) clusters, where 2 ≤ z ≤ 4. Above 1000 m/z these are the most prominent peaks in the mass spectrum suggesting that these clusters form most readily and are relatively stable. This group of clusters grow via the addition of a single I 2 O 5 , and have a consistent number of three ammonia and two iodic acid molecules. Their growth mechanism, by iodine pentoxide with an unchanging number of ammonia and iodic acid molecules, could suggest that once a cluster has sufficient ammonia and iodic acid molecules, the growth of these clusters is driven predominantly by the addition of iodine pentoxide to larger, more atmospherically relevant sizes. As mentioned above, iodine pentoxide can form from the dehydration of two iodic acid molecules. In this case, the nature of the cluster formation via ESI likely results in the dehydration process materializing prior to addition to the cluster since solvent evaporation is one of the mechanisms at play in the production of these clusters with this type of ion source. However, in the atmosphere it is unclear whether this dehydration process would take place prior to addition to the cluster or succeeding its addition. These clusters follow the same growth pattern as the (3, 2, z ) clusters discussed above, where the addition of an iodine pentoxide appears to be the main growth channel. Beyond the (2, 1, 1) cluster, the peak intensities of the (2, 1, z ) cluster series are considerably lower than those of the (3, 2, z ) cluster series, and also decrease in intensity at a much faster rate with each additional iodine pentoxide, suggesting lower stability of the (2, 1, z ) clusters. The (3, 2, z ) series clusters are able to accommodate more iodine pentoxide molecules with less disruption to stability of the cluster as a whole, as these slightly larger clusters have more interaction sites where hydrogen and halogen bonding can take place and can more easily accommodate incoming iodine pentoxide molecules. These trends indicate the presence of ammonia in these clusters is important to supporting further growth via iodine pentoxide.