In order to study the effects of laser induced metal vapour on arc plasma during laser arc double sided welding (LADSW), emission spectroscopy techniques were utilised. The arc plasma in LADSW of 5A06 aluminium alloy was compared with conventional gas tungsten arc welding (GTAW) in this work. The electron temperature and density were estimated by the Saha-Boltzmann equation and the Stark broadening effect. The results indicated that the amount of metal vapour in the arc plasma during LADSW was larger than that of GTAW. Furthermore, the electron temperature of arc plasma in LADSW was lower than that in GTAW, and the electron density of arc plasma in LADSW was higher than that in GTAW near the arc anode. It was shown that the differences in arc properties between LADSW and the conventional GTAW processes were the result of laser induced metal vapour changing the composition of the arc plasma.