The 1 Delta(u), 3d pi component of the lowest ungerade 3d + 4s Rydberg supercomplex of acetylene has been observed by (3 + 1) photon ionization spectroscopy of C2H2 and C2D2. In both isotopic species the vibrationless level of this new electronic state is sufficiently long lived to support rotational structure. From the observed isotopic shift and from the rotational and vibrational parameters determined in the present analyses, this state could be unambiguously identified as the lowest Rydberg state of (1) Delta(u) symmetry, belonging to the 3d complex of acetylene. The weak intensity of this three-photon (1) Delta(u)-(1) Sigma(g)(+) electronic transition is discussed in comparison with the other Rydberg transitions within the same supercomplex. Excited vibronic bands in the same energy region allowed to determine the nu 1 and nu(2) frequencies for some components of this Rydberg supercomplex. The absence of the nearby (H) over tilde (1) Pi(u), 3d delta Rydberg state in the MPI spectra is discussed in terms of predissociation.