The high resolution, single site emission and absorption spectra of magnesium isobacteriochlorin ( MgiBC) in n-octane matrixes at 7 K are reported. Its emission and Q and Soret band absorption regions have been investigated. The vibrational frequencies of the ground and the lowest energy pi pi* singlet excited states were determined from luminescence and excitation spectra, respectively. The emission from MgiBC at 7 K seems to be entirely fluorescence. The luminescence and Q(y) region spectra of the complex are similar, having intense, narrow origin bands followed by relatively weak, but orderly vibrational structure. The Q(x) region does not have a clear origin and exhibits complex vibrational structure that increases in intensity going to higher energy. In the Soret region the individual pi pi* origins are clearly identifiable and some vibrational structure was also observed.