Since the discovery of Ekofisk in 1969 the Norwegian petroleum industry has been an important part of Norway s economical growth and industrial development. Today, Norway is the third largest gas exporter in the world, and the growth in gas production is expected to continue. To ensure continued growth and sustainability of the Norwegian oil and gas industry, recovery rate from existing fields must be increased, and development of smaller, more remote discoveries, must be made profitable. This requires development of new cost effective extraction solutions. Wellstream boosting allows for enlarged output rates, prolonged operation and increased transport distances. The reservoir streams are characterized by a low liquid content of 0-5% on a volume basis, rendering the use of traditional dry gas compressors unfavourable. For this reason, ongoing research aims to develop a compressor capable of operating in wet conditions while maintaining satisfactory performance and operational range. This thesis documents compressor stage instabilities for a low pressure ratio, one-stage air-water multiphase centrifugal compressor. A detailed investigation of instability onset has been carried out, and the effect wet gas operation inflicts on occurring phenomena has been thoroughly documented. Volute pressure buildup appears as an initiating factor for dry gas instability, as a stationary stall region is observed prior to surge onset. Addition of a liquid phase has been found to delay instability onset, which is characterized by a direct transition into modified surge. Possible effects causing a delayed onset has been investigated, including phase segregation in the impeller channels and a delayed pressure buildup in the volute circumference. The previously documented phenomenon of an annulus ring of liquid forming at impeller inlet prior to instability onset in wet conditions has been further investigated, showing a significant impact of labyrinth leakage on detected inlet flow phenomenon.