An experimental study was carried out to investigate fluidelastic instability in finned tube bundles in normal and parallel triangular arrays. Three arrays of each geometry type were studied experimentally: two arrays with serrated, helically wound finned tubes of different fin densities, and a bare tube array with the same base diameter as the finned tubes. All six tube arrays studied had the same tube pitch. The finned tubes under consideration were commercial finned tubes typically used in the fossil and process industries. For the purpose of the present investigation, the concept of “effective diameter” of a finned tube, as previously used to predict vortex shedding, was used to compare the finned tube results with other finned tube results as well as the existing bare tube world data. The experimental results for the triangular arrays show that the fin’s structure strongly influences the fluidelastic stability of finned tube bundles and the fin pitch is demonstrated to reduce the difference in the stability threshold between the tube array geometries as the fin density increases. Overall, the effect of serrated fins on fluidelastic instability is very complex and array geometry dependent, stabilizing some arrays and destabilizing others. Clearly, the effect of fins cannot be accounted for by the simple use of an effective diameter of an equivalent bare tube.