The np gerade Rydberg states of acetylene were analyzed using two-photon resonance fluorescence excitation spectroscopy in the 72 000-93 000 cm(-1) energy region. The np pi(1)Sigma(+)(8); and np pi(1)triangle(g) Rydberg series (n = 3-5) were identified in the fluorescence excitation spectrum measured by monitoring the C-2 d(3)Pi(8)-a(3)Pi(u) Swan system. Some vibronic bands were assigned to the np pi(1)Delta(g)-X-1 Sigma(+)(8); transition on the basis of rotational analysis. The 5p sigma(1)Pi(g) state was observed, which is the first such observation in an np sigma(1)Pi(g) series. Rotational analysis of the 5p sigma(1)Pi(g)-X-1 Sigma(+)(g) transition showed e/f-symmetry dependent predissociation of acetylene in the 5p sigma(1)Pi(g) state. The 0(0)(0) band of the deuterated acetylene (C2D2) 4p pi(1)Sigma(+)(g) - X-1 Sigma(+)(g) transition exhibits an atypical structure, which was satisfactorily reproduced by a simple model of quantum interference between the discrete and quasi-continuum states. The predissociative lifetimes of the np pi gerade Rydberg states were estimated from the spectral profiles. The predissociation mechanism of acetylene in the Rydberg states is discussed.