In this Letter, we investigated the dimerization behavior of the red-absorbing oxazine derivative MR 121. Thereby a new method to determine absorption spectra of pure non-fluorescent H-type dye dimers is presented. Therefore, the amino acid lysine is doubly labeled with the fluorescent dye in order to obtain an absorption spectrum mainly caused by dimers. By subtracting the fraction arising from monomeric dye, the dimer spectrum is generated. The extinction coefficient at absorption maximum (608 nm) was estimated to be 86000 +/- 2000 M-1 cm(-1) per dye. Furthermore, the equilibrium constant K-D for the dimerization reaction is determined to be 3300 +/- 300 M-1. (c) 2005 Elseiver B.V. All rights reserved.