This paper describes the pressure distribution of turbulent airflow through a flat duct containing one-dimensional symmetric sudden expansion in cross-sectional area encountered frequently in plate heat exchangers and other devices of practical importance. The pressure distributions of turbulent asymmetric airflow through constant width flat ducts having modest expansion ratios of 2, 2.5 and 3, aspect ratios of the expansion of 0.40, 0.26 and 0.20 and modest area ratios of the expansion of 1/3, 1/2.5 and 1/2 were investigated experimentally. The axial and vertical pressure distributions of the turbulent asymmetric flow with relatively small Reynolds numbers (Re-Dh = uD(h)/v = 12 000,4887 and 4125) in downstream cross section of the duct were analyzed. The variations of pressure coefficient with local Reynolds numbers were also investigated. Turbulent flow in flat ducts with symmetric sudden expansion has not sufficiently been studied contrary to axisymmetrical or two-dimensional case for which a large numbers of papers can be found in the literature. This experimental study fulfills this gap and shows that the flow is asymmetric as in a two-dimensional case. It also brings interesting data for numerical simulations for the presented flat duct configurations. These data,concern the axial and vertical distribution of pressure coefficient, the axial velocity in the duct and Reynolds numbers based on the hydraulic diameter, the step height or the local axial distance of the duct. The results obtained complement and extend the previous results, and show that the expansion ratio and Reynolds number affect the flow field. (C) 2004 Published by The Japan Society of Fluid Mechanics and Elsevier B.V. All rights reserved.