[en] The trace polymer and additives in used stripper solutions were analyzed by a combination of Py-GC/MS and ESI-MS. In the comparison of the pyrolysates produced by the pyrolysis of the pure stripper and photoresist at 500 .deg. C, the presence of novolac polymer in the used stripper was confirmed by the presence of the characteristic peaks of its pyrolysates, such as those of the methylphenol, di-methylphenol and methylenebis(methylphenol) isomers. The intact trace polymer was measured by ESI-MS, which showed the distribution of oligomers at intervals of 120 Da, indicating di-methylphenol to be the repeat unit. Additional MS/MS measurements demonstrated that the end group is methylphenol and the repeat groups are di-methylphenol. Some modified oligomers caused by the methylation or di-methylation of the repeat unit were also identified. Although the polymer is only present at a trace level in the used stripper, these combined analytical methods provided the means to qualify the stripper solution through the identification and structural determination of the polymer