OD EPR spectrum was recorded in C2H2, for triplet levels coupled by intramolecular interactions to levels of the adjacent (A) over tilde (1)A(u) singlet state, excited into individual (A) over tilde (1)A(u), 3(2)K(2), (J' = 4) rotational level. Spectroscopic parameters of the triplet states were estimated by fitting a theoretical model to the experimental spectra. The values obtained were discussed and compared to spectroscopic parameters of other molecules. The presently published values of fine and hyperfine constants represent the first and only set of such data available. Ab initio analysis of values of the hyperfine constants of the (a) over tilde B-3(u), (b) over tilde (3)A(u), and (c) over tilde B-3(u), states allowed to make assignment of the spectroscopic parameters measured to specific triplet states, and to determine the relative contributions of these states to the mixed S-T levels. Time-resolved experiments demonstrated the sensitivity of the S-T conversion dynamics to external magnetic and microwave fields. (C) 2004 Elsevier B.V. All rights reserved.