A miniature reaction cell was developed in order to study the influence of gas phase components on reactive wetting and dewetting processes. Small silicon droplets on fused silica can be observed in situ at 4000 frames per second under reaction conditions by a microscope equipped with a high speed camera. Additionally, ex-situ investigations of etch profiles originating from the reactive wetting process are conducted by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Under certain reactive conditions large dynamic spreading modes exhibiting a new type of droplet instability are observed. Some spread-out droplets suddenly disrupt and decay into a ring of smaller droplets. Each of these then spreads again and disrupts into a circular array of even smaller droplets forming a ring-shaped structure. Whole cascades of decay can be traced by means of the etch profiles found on the substrate. The results are discussed within a simple thermodynamic model that relates the changes in the oxygen chemical potential to the changes of solid-liquid interface tension.