We study the 3D flow in square-square sudden contractions under low Reynolds number flow conditions. Visualizations of the flow patterns were undertaken using streak line photography, and detailed velocity field measurements were conducted using a Particle Image Velocimetry technique. We analyze the effect of contraction ratio on the flow patterns observed with a Newtonian and a Boger fluid. For the Newtonian fluid, the experimental results obtained are compared with numerical simulations performed using a Finite-Volume method, and excellent agreement is found. For the viscoelastic case, we observe complex flow patterns that are dependent on the contraction ratio. For low contraction ratios strong divergent flow is observed upstream of the contraction, which is consistent with the findings of Alves and Poole [J. Non-Newt. Fluid Mech. (2007) In Press. doi:10.1016/j.jnnfm.2007.04.003]. On the other hand, for high contraction ratios no upstream divergent flow is observed, except in the vicinity of the re-entrant corner, where a localized atypical flow is observed. For all contraction ratios studied (CR = 2.4, 4, 8 and 12), increasing the Deborah number leads to a strong elastic vortex enhancement upstream of the contraction, until a periodic complex flow is established at higher flow rates.