This work discusses energy absorption mechanisms in CO2 laser welding and how they are affected by changes in the process speed. Two main energy absorption processes govern the welding interaction: 1. Fresnel absorption at the keyhole walls. 2. Absorption by the partially ionised metal vapour (or plasma) in the keyhole (laser energy absorbed in this way is re-radiated or conducted to the keyhole walls). A theoretical model of these absorption mechanisms has been developed and shown to agree closely with experimental results. Fresnel absorption has been identified as being dominant over plasma absorption and becomes even more influential as welding speeds are increased.