Electron spin echo envelope modulation (ESEEM) has been studied at zero and low magnetic fields (B </= 100 G) by means of optically detected magnetic resonance. Qualitative differences of the ESEEM effect for an electronic triplet state (S = 1) under low-field and high-field conditions are observed and discussed. They are related to the different properties of the total spin angular momentum operator S and the hyperfine interaction at zero and high magnetic field. The novel method was applied to the photoexcited triplet state of acridine-d9 in a fluorene-h10 matrix. An analysis of the observed nitrogen quadrupole splitting is done by a quantitative description of the ESEEM effect in zero field. Copyright 1998 Academic Press.