Quantum-chemical calculations have been performed to obtain ionization potentials and electron affini- ties of impurity molecules detected experimentally in tetracene crystals. Resulting data with a model of the disorder of charge-transfer integrals have been applied in transfer-matrix method calculations of localization lengths on a triangular two-dimensional lattice representing tetracene crystal. Neat systems (with purely off-diagonal disorder) exhibit very long localization lengths at the band center. Binomial diagonal disorder reduces localization lengths of states in the middle of the band; this effect is the largest for impurities with on-site energies most different from tetracene. Additional diagonal disorder (bino- mial or Gaussian) may increase correlation lengths for strongly localized states at the band edges. The net effect of the diagonal disorder close to band edges results from interplay of impurity concentration and its on-site energy.