The flow characteristics of a single and the two side-by-side spheres immersed in a uniform flow for Re = 5 x 10(3) are studied qualitatively and quantitatively employing dye visualization and a particle image velocimetry. The current study focuses on the analysis of the flow behavior using instantaneous and time-mean flow data in plan-view and cross-sectional planes. Flow features and distributions of velocity vectors, V*, at the critical locations; patterns of velocity fluctuations, u* and nu*; contours of sectional streamlines, ; patterns of vorticity, omega*; and correlations of Reynolds stress, are interpreted. The gap ratios between two side-by-side spheres varying within the range of 1.0 <= G/D <= 2.50 influence the instability of flow structures substantially.