The flowing movement of lightly colored polyisobutylene (PIB) primary sealant into the interpane space of in-service insulating glass units (IGUs) is a phenomenon we have encountered at multiple locations in the United States. This behavior has resulted in the accumulation of visible deposits within the exposed field of the IGU in excess of the dimensional limits recommended by insulating glass manufacturers and industry standards. Such conditions create aesthetic and potential long-term durability concerns and attribute to the failure of the PIB material to perform in a durable manner. Flow of the PIB material has been observed to occur when standard practices and materials were used during IGU fabrication and glazing operations and when the IGUs were exposed to common environmental conditions. This paper discusses contributing factors to the PIB flow phenomenon we observed. The data garnered from observations and analysis revealed a correlation of PIB flow with IGU solar exposure and consequential effects on material properties. Chemical analysis of PIB samples removed from affected in-service IGUs indicates physical changes occurred that correspond with changes in the material at the molecular level. Locations of PIB flow correlated well with areas of increased solar exposure and isolation from interior conditioned air.