Protonolysis, complex-formation with amines, ab initio molecular orbital calculations with 3-21G basis set, redox potentials, and spectroscopic features of 1-phenylthieno[3,4- d]borepin ( 1) and 1-phenylthieno[2,3- d]borepin ( 2) were examined. The compound 1 was shown to be more labile than 2, which give us a guide for construction of stable heteroaromatics. Absorption and fluorescence spectra of 1 and 2 revealed the potential ability of boron heterocycles in the field of the organic optical materials.