Abstract—The triplet states of some acridine dyes (proflavine, acriflavine, acridine yellow, acridine orange, ethacridine (Rivanol), benzoflavine), dissolved in a dimethylformamide/water matrix were investigated at 90°K by ESR. Zero‐field splitting parameters were calculated from the ESR spectra. The relationship between the zero‐field splitting and the distribution of the spin‐density of the electrons of the triplet state of the acridine derivatives is d