The effect of heat on coherent vortex structures shed from a cylinder is investigated numerically and experimentally for ReD ~ 75 and RiD = 0 − 1.2. For the experimental investigation a High Resolution Particle Velocimetry (Hi-Res PV) method is used. For the numerical investigation a Spectral Element Method is used. Both investigations show that for relatively small heat input (RiD < 1) the vortex street deflects in the direction of the gravity force. Besides, subsequently shed structures show a relative rotation around each other, causing an unstable vortex street. For higher heat input the vortex structures itself become unstable. Three-dimensional structures are formed inside vortices shed from the upper half of the cylinder, occasionally leading to an early breakdown of the vortex street.