Abstract The triplet state of the primary donor of photosystem II particles prepared from a mutant of Chlamydomonas reinhardtii has been studied at 1.2 K with absorbance-detected ESR in zero-magnetic field (ADMR). Two sets of resonances with slightly different zero-field splitting parameters | D| and | E| were observed, | D| = 285.5, | E| = 38.8 and | D| = 288.8 × 10 −4 cm −1, | E| = 42.2 × 10 −4 cm −1, respectively. Both sets of | D| and | E| values are close to those found for P T-700, as are the sublevel decay rates k x = 930 ± 40, k y = 1088 ± 50 and k z = 110 ± 5 s −1. The AMDR-detected triplet-minus-singlet absorbance difference spectrum of P T-680 is very similar to that of P T-700 and closely resembles that of covalently connected Chl a dimers in vitro. We conclude that P-680 is a Chl a dimer whose general structure is similar to that of P-700.