An experimental and theoretical study was carried out to investigate the keyhole collapse and pore formation in keyhole laser spot welding; based on this a process theory was developed. Laser spot welding experiments were carried out mainly in liquid zinc (to simplify the investigation); X-ray imaging was used to observe keyhole behaviour i.e. collapse following switching off of the laser beam. A semi analytical mathematical model was used to simulate keyhole collapse taking into account a number of physical processes. The experimental results and mathematical analysis were used to determine the different process phases occurring during keyhole collapse, also indicating the timescales involved: post-vaporisation, excess keyhole vapour relaxation flow, inertia driven collapse, bubble contraction, recondensation, and rising and resolidification.