This paper reports on experimental investigations of fluidelastic instability in tube arrays subjected to air cross-flow. The vibration behaviour of one flexibly mounted tube within otherwise fixed bundles with different geometries has been examined. The applied experimental apparatus allows to realize four tube patterns (normal and rotated triangular, in-line and rotated square) which are usual in heat exchangers. The influence of the pitch-to-diameter ratio is investigated for the triangular arrays. The free tube has a linear iso-viscoelastic suspension. This means, neither stiffness nor damping of the tube mounts is dependent on the amplitude and the direction of motion. Using eddy current dampers the mass-damping parameter of the free tube can be varied continuously over a wide range (\left(11 \cdot 7 \leq \delta_{r} \leq 500\right)). Adjustable tube mounts of the flexible cylinder enable to maintain the ideal tube array geometry during all experiments. Results are presented showing the stability boundaries of the flexibly mounted cylinder positioned in each of the first three or four rows of the different arrays. These stability thresholds of the equilibrium position of the free tube depend on the array pattern as well as on the tube distance. This shows the limitations of the standard stability diagram, common for different pitch-to-diameter ratios.