An investigation is made of the melt-pool formed during keyhole welding with a cw CO2 laser on thin plate mild steel. The aim of the study is to analyze the dynamics of the melt-pool and keyhole in order to provide information on the causes of instabilities found during high speed welding. Such problems found during high speed welding include humping, keyhole failure, and surface tension driven melt-pool instabilities. The effects of varying laser power (2 to 4 kW), traverse speed, shroud gas, gas delivery angle, and plate thickness were studied. The methods used included various high speed camera techniques. Two high speed cameras are used, a high speed video camera at a frame rate of 1,000 frames per second and a high speed gated camera used in conjunction with a frame grabber capable of gate speeds as low as 25 ns and freeze frame multi-imaging. The high speed video system was used to gather information on the gross melt-pool characteristics, e.g., shape, length, width, and any other slow changes present (of the order of 100 Hz). It is hoped by correlating these results with theory that an insight into high speed behavior will be obtained.