This paper presents an experimental study of the structural dynamics and the wake vortex modes of a lightly damped, elastically mounted cylinder undergoing vortex-induced vibration (VIV) in the transverse direction. The cylinder is neutrally buoyant with the mass ratio m* = 1.0, and has a low damping coefficient zeta=0.0173. The influence of variation in the free-stream velocity (corresponding to the reduced velocity range 1.53 <= U* <= 6.62, or the Reynolds number range 3000 <= Re <= 13,000) on the structural dynamics of the cylinder is examined, in conjunction with detailed flow measurements around the cylinder using digital Particle Image Velocimetry (PIV). The cylinder is stationary at U*= 3.05 (or in synchronization regime). The vibrating cylinder exhibits the so-called "soft" synchronization phenomenon that the vortex shedding frequency is synchronbus with the structural vibration frequency (or response frequency) of the cylinder, rather than with the natural frequency of the system. The measured data for the cylinder undergoing VIV fall within the initial branch and the upper branches, and agree well with published data on free vibrating cylinders at similar low mass and damping. The transition from initial-to upper-branch is characterized by a switch of vortex formation mode from the classical 2S mode to the newly-discovered 2P(O) mode by Morse and Williamson (2009, Journal of Fluid Mechanics, Vol. 634, pp. 5-39) based on a forced vibrating cylinder. The distinction between the 2S and 2P(O) modes is vivid in terms of length- and velocity-scales of the shed vortices, and is also reflected by quantitative characterization of near-wake flow statistics, including the mean and turbulence properties. (C) 2016 Elsevier Ltd. All rights reserved.