We report the results of a recent MHI study in which three important aspects of fluidelastic instability were considered: (i) two-phase fluid damping and added mass under prototypical conditions, (ii) the nature of unsteady fluid forces leading to fluidelastic instability, and finally (iii) the fluidelastic instability mechanism itself. This paper is the first in a three-part series reporting on the findings of the comprehensive study. Tests have been conducted to determine two-phase flow damping under prototypical high temperature and pressure conditions, up to 5·8 MPa at 273°C. The test array was of an in-line geometry. Two separate arrays were tested, a standard configuration with flow normal to the tube axis and an array inclined at 30° to the flow. Measurements were conducted for various pressures (and correspondingly temperatures), void fractions, phase flow velocities as well as tube location within the array. Damping was nominally higher for the inclined array. Tubes at the array extremities were found to experience the highest damping levels. This is attributed to entrance and exit effects. A distinct difference between drag- and lift-direction damping was observed. Tube added mass showed a quasilinear variation with void fraction. Added mass values were found to vary significantly from computed values assuming homogeneous fluid properties.