Apparatus for temporarily holding flexible sheets and conveying them progressively through an image transfer station, e.g. in a xerographic printer comprising a rotatable sheet holder (30) having two segmental peripheral portions (34,35) which have inter-meshing teeth and can be relatively angularly displaced for varying the arcuate length of the sheet supporting surface (39) formed by such portions. For gripping the leading and trailing margins of a sheet (38) and holding it taut against the surface (39) there are leading and trailing clamps (58, 48) which are likewise relatively angularly displaceable so that they can operate at the leading and trailing ends of the arcuate support surface (39) whatever be its adjusted size.