It has been found that rotational levels of the ungerade 3v'(3) + v'(6) vibrational state of acetylene in the (A) over tilde (1)A(u) state split into two or more levels, while those of the 3v'(3) + v'(4) state indicate no splitting. The level splitting is attributed to the S-1-T-n coupling which has been proved by observation of the Zeeman quantum beat in the fluorescence decay from the split level under a weak magnetic field. From the present finding that the S-1-T-n coupling occurs only in the combination state of the in-plane v'(6) mode with the 3v'(3) state, it is concluded that the S-1 state couples with the T-n state in a C-2h planar geometry. (C) 2001 Elsevier Science B.V. All rights reserved.