The present article describes the development of an analytical method for the determination of 13 perfluorinated alkyl substances (PFAS), as well as its application to real sewage sludge samples to confirm the presence of these compounds. The isolation of the analytes was performed by agitation, sonication and centrifugation techniques, followed by EnviCarb cleanup and weak anion exchange solid-phase extraction. Sensitive and selective determination was carried out by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Six mass-labelled internal standards were used to ensure the accuracy of the analytical results following isotopic dilution method. Several mobile phases (acetonitrile, methanol, mixtures of both and water with ammonium acetate or acetic acid) have been tested to reach the best resolution and reproducibility results. Other parameters related to MS/MS conditions were optimized. The reliability of the method was confirmed by the evaluation of linearity (R(2) = 0.995-0.999), accuracy (84-99%) and injection repeatability and reproducibility (relative standard deviation below 19 and 23%, respectively). Limits of detection ranged from 0.007 to 2.217 pg. Recoveries show values higher than 80% for most of the target compounds. The application of this method to twenty real samples demonstrates its efficiency and accuracy, as well as provides for the first time to our knowledge, PFAS levels in sewage sludges from Spain.