The effect Of CO2 on the welding kinetics for a PS colloidal crystal was determined by insitu measurement of Bragg diffraction and by ex-situ SEM microscopy. The welds grow tangentially from the points of contact of the spheres in all directions transforming the spheres into a continuous film. The dissolved CO2 provides the mobility for the polymer chains at the interface needed for welding, which removes the energetically unfavorable interfacial area between the polymer and CO2. At high pressures where CO2 is relatively incompressible, the relaxation time for welding increased by 6 orders of magnitude with an increase in thermal energy, and a glass transition temperature was observed. At constant temperature, an anomalous increase in the welding rate by 1-2 orders of magnitude with a decrease in pressure was observed in the region where CO2 is highly compressible. It is likely that this increase in welding dynamics is influenced by the excess CO2 in the interfacial region.