Experiments were conducted on two-phase flow in laterally ribbed rectangular ducts. Air-water adiabatic flow at atmospheric pressure and room temperature was driven through a 3.6 m long rectangular ribbed test section with cross-section of 100 x 50 mm. To investigate the effect of rib thickness and pitch on flow pattern diagrams and transition boundaries, nine various rib arrangements were implemented with thicknesses of 2, 4 and 8 mm and pitches of 50, 60, and 80 mm. Unlike non-ribbed rectangular duct, lateral rib arrangement did not allow any stratified flow to occur. However wavy, plug and slug flows were parallel in both flow conditions, rib existence caused explicitly coarser pattern shapes. Increasing the rib thickness, while keeping the pitch constant, results in different flow patterns to occur as well as dramatic changes in boundaries positions and shapes. On the other hand, as pitch shifts up at a constant rib thickness, one can notice the duplication of almost identical flow patterns and their boundaries however, boundary values undergo tangible changes. Consistent attention was paid to conditions under which wavy pattern zone extends while intermittent flow zones were avoided. Studies concerned ribbed duct are of major applicable value to designing and enhancing heat transfer systems. (C) 2013 Elsevier Ltd. All rights reserved.