The arylation of readily available benzo[b]quinolizini-um-9-boronic acid with carbocyclic or heterocyclic bromoarenes in the presence of a palladium catalyst [Pd(dppf)Cl 2 ·CH 2 Cl 2 or Pd(PPh 3) 2 Cl 2 ] gives the corresponding cationic biaryl products in yields of 15–81%. All aryl-substituted benzo[b]quinolizinium (acridizinium) derivatives exhibit a long-wavelength absorption maximum between 409 and 422 nm in water, which change margin-ally in different solvents (Dl <10 nm). As the only exception, the 9-(N,N-dimethylaminophenyl)benzo[b]quinolizinium exhibits pro-nounced solvatochromic behavior (H 2 O: l abs = 422 nm; MeCN: l abs = 474 nm; CH 2 Cl 2 : l abs = 507 nm) due to the strong donor– acceptor interplay in the cationic chromophore. Depending on the donor strength of the aryl substituent, the benzo[b]quinolizinium derivatives exhibit fluorescence bands in water with long-wave-length maxima between l fl = 452 nm (R = phenyl) and 529 nm (R = 4-methoxyphenyl). The non-fluorescent 9-(N,N-dimethylami-nophenyl)benzo[b]quinolizinium may be employed as a pH-sensi-tive light-up probe (pH 2.8–4.8), since its emission intensity increases by a factor of 130 upon protonation.