The low absorption presented by thermoplastic films to 10.6-μm CO2 laser radiation makes the engineering use of welding parameters, predicted by models developed for thicker thermoplastics, very difficult. A new theoretical model is developed describing the temperature distribution in thin thermoplastic material during the laser welding process. The heat conduction equation is solved analytically by the Green function method and heating and cooling thermal stresses are taken into consideration. Engineering parameters predicted by the model are applied to lap welding of high- and low-density polyethylene and polypropylene samples, both transparent and white, with thicknesses between 10 and 100 μm, and experimentally validated. This validation is also accomplished by comparison with the measured temperature through the use of two diagnostic methods: schlieren interferometry and photothermal deflection spectroscopy. The first of these methods, combined with direct observation of Mie scattering, also puts in evidence the absorption of about 30% of the incident energy due to plasma formation in the air above the interaction interface. This plasma ignites after the initial release of chunks of material during the first moments of interaction. Proper modeling, and the introduction of a reflective substrate under the samples, allows an increase in process efficiency and the achievement of lap welding speeds up to 14 m s–1 with this new transmission welding technique.