A temporal linear stability study was performed for a gas-liquid annular flow configuration under microgravity conditions. Data used to validate the modeling includes that generated by Texas A&M as well as all the other known data in two-phase flow under reduced gravity conditions. Following a discussion of theoretical considerations on the growth rates of different instabilities, it is shown that given the fluid properties, pipe diameter and phasic flow rates, one can predict with a high level of confidence the flow regime in the pipe. Acceptable confidence levels (similar to 80%) are achieved when one differentiates between slug, slug-annular, and annular flow. Higher confidence levels (similar to 90%) are found when one differentiates between slug and annular flow by merging the annular and slug-annular categories.