Alternating PbSt 2, Pb(St-d 35) 2, and poly(t-butyl methacrylate) Langmuir-Blodgett (LB) films were prepared and the splitting of CH 2 and CD 2 scissoring mode was monitored. The mechanism of defect formation upon thermal treatment of the fatty acid salt LB films was investigated by Fourier transform infrared spectroscopy and X-ray reflectivity. The insertion of polymer layer improves the stability of LB films. The less reduction of the band splitting for the fatty acid salt layer below the polymer layer supports the ablation as the major mechanism taking place during the thermal treatment.