Following optical excitation of the (A) over tilde (1)A(u), 2v(3)('), K-a=0 and 2 states of acetylene under collisionless conditions, population is transferred to the continuum spectrum of the ground state (the dominant process), to neighboring triplet "dark" manifolds as well as to low vibrational states in the ground state (as fluorescence). These states undergo the singlet-triplet crossing reversibly and intramolecular relaxation from the triplet manifold is not significant. Upon applying H in the presence of collisions we observe shortening in the fluorescence decay times and reduction in the amplitudes of decay signals. The magnetic quenching originates from the enhanced participation of triplet levels in the collisional deactivation, caused by the Zeeman mixing of triplet fine and hyperfine levels. The ratio of the singlet to triplet collisional cross sections for individual rotational states of sigma(T)/sigma(S)approximate to2, is estimated based on the response of the fluorescence decays to the external magnetic field. (C) 2003 American Institute of Physics.