The components of the 5s + 4d Rydberg complex of C2H2 and C2D2 have been studied by (3 + 1) photonionization spectroscopy and fragment fluorescence excitation spectroscopy following synchrotron radiationVUV excitation. Thanks to the remarquable spectral resolution of the new synchrotron beam line SU5 atSuper-ACO (12 mÅ in the range 248−65 nm), rotationally resolved spectra of Rydberg states of acetylenecould be observed through their photofragment visible fluorescence. Unlike the lower 4s + 3d Rydberg complex,all Rydberg components of this complex exhibit rotational line widths typical of a 1−10 ps predissociation.The 1Φu 4dδg and 1Δu 4dπg components have been characterized for both isotopic species. The relative electronicband intensities of the REMPI spectra have been interpreted within a semi-united atom model and by takinginto account predissociation.