Waviness or fluting occurs in the hot-air drying oven of the heatset web offset (HSWO) printing press and reduces the visual quality of the print. In order to evaluate a theory that waviness results from stretching of the material in one direction while being prevented from shrinking in the perpendicular direction, the reaction of a paper web under tension to moisture was observed. A paper web was stretched under tension using three rolls, the spacing of which could be varied. The web was wetted using humid air of 60-85% RH and the web observed using the STFI OptiTopo optical topography technique. Images were obtained using a charge coupled device (CCD) camera. Two 70gsm medium weight coated (MWC) papers were used, one prepared using a gap former with online coating and calendering, the other prepared with a hybrid former followed by online coating and offline calendering. The paper tensile properties were determined. Although no ink was involved, the typical waviness pattern was seen to develop. Higher moisture uptake resulted in a greater waviness. The two papers had similar bending stiffness, but the paper with the highest tensile stiffness showed the lowest waviness