An examination of two-phase flow patterns and pressure drops in single and double helically ribbed circular tubes has been performed. The present studies were conducted in both a 2.54 and 5.1 cm circular tube using low pressure air-water mixtures maintained at room temperature. Ribs of a circular cross section were utilized with heights of 0.32, 0.64 and 1.27 cm. Helical twist ratios of 1.2, 2.1, 2.3 and 2.5 were used. Results indicate that swirling annular flow is seen at low qualities once a minimum liquid velocity is exceeded. Therefore, it is expected that an enhancement of the critical heat flux would also occur above this minimum liquid velocity. Dimensionless correlations have been devised for the various flow pattern transitions which were observed.