The effects of streamwise aspect ratio and Reynolds number on the separated shear layer and near wake of rectangular cylinders in uniform flow are investigated experimentally using a particle image velocimetry system. Four length-to-height ratios (AR = 1, 2, 3 and 4) were examined at Reynolds numbers (based on freestream velocity and cylinder height) of 3000, 7200, 14700 and 21000. The results show that the separated shear layer is either shed directly into the wake region (AR1 and AR2) or reattaches onto the cylinder (AR4), regardless of the Reynolds number. Meanwhile, a transitional regime occurs for AR3 where mean flow reattachment on the cylinder is highly dependent on the Reynolds number. The peak magnitudes of the Reynolds stresses, turbulent kinetic energy, turbulence production and its transport are highest for AR1 owing to stronger vortex shedding. Aspect ratio and Reynolds number also have significant effects on shear layer transitioning from laminar to turbulence but the transition lengths, when normalized by the corresponding value at Re = 3000, follow a universal exponential decay law. The wake characteristics, including the recirculation length and wake formation lengths, are independent of Reynolds number for AR1 but decrease with Reynolds number for the longer cylinders, while AR2 shows the largest values. The probability density functions, and joint probability density functions are used to examine the effects of Reynolds number on the fluctuating velocities and momentum transport in the shear layer and near-wake region.