High-resolution optical spectroscopy and ESR are applied to the guest triplet (T 1) states in isotopically mixed phenazine crystals at helium temperatures. The T 1 ← S 0 0-0 transitions in P- h g in P- d g and P- d g in P- h g are investigated using laser-excitation spectroscopy and phosphorescence emission spectroscopy. The P- h g guest aggregate spectra at 1.6 K are studied as a function of the guest concentration up to 5%. Due to the high resolution achieved monomers and three dimer lines are spectraly resolved. The quantitative analysis allows a more detailed assignment of the triplet exciton band structure of phenazine. ESR of the guest triplet state of P- h g in P- d g is investigated at helium temperature for guest concentrations between 0.5 and 5%. ESR of the AB guest dimer is observed only at c G = 2% and only at Q-band but not at X-band frequencies. The explanation of the experimental findings is based on the model of the coherent dimer excited state.