Nanosecond transient expansion and contraction dynamics of poly(methyl methacrylate) (PMMA) and poly(methyl acrylate) (PMA) films doped with pyrene as a sensitizer are induced by an excimer laser irradiation, which was studied by applying nanosecond time-resolved interferometric technique. At 150 mJ/cm(2) polymer films of similar to 2 mu m thickness showed a rapid expansion of about 250 nm and a following slow contraction, resulting in no permanent etching on the surface. The contraction behavior depended on thermal properties of substrates, and the maximum value of the transient expansion showed different fluence dependence for PMMA and PMA films. It is considered that heat energy generated by the excimer laser irradiation via photothermal conversion process of pyrene is transferred from doped pyrene to polymer matrix, which results in temperature elevation. The difference in expansion behaviors between two polymer films was well interpreted in terms of the glass-rubber transition of the polymer films and of related molecular motions in polymer chains.