Since the shutdown of the San Onofre Nuclear Generating Station (SONGS), the question of the possibility of in-plane fluidelastic instability in the steam generators was raised. In a step to analyse the stability of the APR1400 steam generator, this paper reports on an experimental fluidelastic instability study in single phase (water) and two-phase (air–water) cross flow of the vertical tube bank in the steam generator. The study concerns the vibration behaviour of single flexible tube with an otherwise rigid array as well as fully flexible array. The tube pattern analysed is the rotated triangular array (60°) having a pitch spacing (P/D) of 1.33. This spacing is generally more confined than what is usually seen in the literature. Adjustable tube mounts allow the tubes to vibrate only in one direction, whether in the streamwise or transverse to the flow. The stability threshold in the transverse direction of such a tightly spaced rotated triangular array is somewhat lower than the other layouts. Damping of this rotated triangle array reaches its maximum value at a void fraction near 80%. Fluidelastic instability critical velocity in the streamwise direction was found to be much higher than the range of velocities implied by the ASME design guideline, which implies relatively high Connors constants. This was observed in the case of fully flexible array only, where cross-coupling between the tubes is significant. However, A single flexible tube did not undergo instability, which indicates that the streamwise instability is governed by the fluid stiffness mechanism.