The velocities, turbulence intensities, Reynolds shear stresses, and turbulent kinetic energies of the flow fields around two square cylinders in a tandem arrangement were investigated using particle image velocimetry (PIV). The experiments were made for the spacing between the two cylinders ranging from s/D = 0.5 to 10.0 and two Reynolds numbers of 5,300 and 16,000. The results showed that the flow patterns at $s/D{le}2.0$ were drastically different from those at $s/D{ge}2.5$ for both Reynolds numbers. The sudden change in the flow patterns depended on the reattachment of the shear layer separated from the upstream cylinder.