An experimental investigation is conducted on the ow around tandem cylinders for ranges of diameter ratio d/D = 0.25–1.0, spacing ratio L/d = 5.5–20, and Reynolds number Re = 0.8 × 104–2.42 × 104, where d and D are the diameters of the upstream and downstream cylinders, respectively, L is the distance from the upstream cylinder center to the forward stagnation point of the downstream one. The focus is given on examining the effects of d/D, L/d and Re on Strouhal number St, flow structures and fluid forces measured using hotwire, particle image velocimetry (PIV) and load cell measurement techniques, respectively. Changes in d/D and L/d in the ranges examined lead to five flow regimes, namely lock-in, intermittent lock-in, no lock-in, subharmonic lock-in and shear-layer reattachment regimes. Time-mean drag coefficient (CD) and fluctuating drag and lift coefficients (CD' and CL′ ) are more sensitive to L/d than d/D. The scenario is opposite for St where d/D is more prominent than L/d to change the St. The detailed facet of the dependence on d/D and L/d of CD, CD′, CL′ and St is discussed based on shear-layer velocity, approaching velocity, vortex formation length, and wake width.