Abstract: A mobile floating surface skimmer is adapted to float in a body of liquid having debris and/or contaminants at the surface. The skimmer comprises a vessel having an inlet through which liquid from the surface of the body of liquid can flow into the vessel together with the debris and/or contaminants. An outlet is adapted to be connected to a pump for removing liquid, together with the debris and/or contaminants from the vessel. Further included is a valve for controlling a flow of liquid into the vessel in response to the level of liquid in the vessel so as to maintain the inlet near the surface of the body of liquid, wherein the valve is provided with a delayed response so as to cause the skimmer to oscillate vertically in the body of liquid so the debris is prevented from becoming lodged at the inlet. Further, the invention includes a method of skimming debris from the surface of a liquid.