A systematic investigation of the orthorhombic borides LiAlB 14, MgAlB 14 and ErAlB 14 was carried out. In all cases several indirect allowed optical interband transitions with phonon emission were derived from transmission measurements in the absorption edge range. For LiAlB 14 the temperature dependence between 22 and 293 K was determined. The results are compatible with the interband photoconductivity and the electroabsorption. Despite the identical basic structures of the icosahedral boron network, the interband transition energies of the orthorhombic borides depend on the chemical composition. As for other icosahedral boron-rich solids, there are edge tails with considerable absorption levels extending to lower energies, at least in MgAlB 14 and ErAlB 14, obviously evoked by the metal atoms. ErAlB 14 is a one-dimensional conductor or a one-dimensional high-conductivity semiconductor with a distinct plasma edge close to 2000 cm −1. The IR phonon spectra in the spectral range between 2000 and 1200 cm −1 show the vibrations of the light single atoms. The phonon spectrum at 1200 cm −1 or less, belonging essentially to the intra-icosahedral vibrations, is closely related to the α-rhombohedral boron structural group. Phonon quenching as a result of electron-phonon interaction on the icosahedra occurs as the atomic number of the metal atoms is increased. The carrier type depends on the transfer of electrons from the metal atoms to the structure. In the case of MgAlB 14 the Seebeck coefficient is of the order of −6500 μV K −1 making it of interest for thermoelectric applications.