The photoelectron (PE) and X-ray absorption (NEXAFS) spectra of gaseous diethynylthiophene (DET) at the C 1s and S 2p core levels are presented and discussed. The NEXAFS spectra have been obtained in the total-ion-yield mode. The ionization energies have been determined for C Is and S 2p spectra by DeltaSCF calculations, while the NEXAFS spectra, at both the C K-edge and S L-edge, have been Simulated by STEX calculations. Good agreement between experiment and theory is generally observed. A strong perturbation of the electronic structure of thiophene at the alpha- and beta-carbon atoms, induced by the two ethynyl substituents, is inferred by the analysis of the spectra.