The microwave characteristics of the BaTi4O9-BaPr2Ti4O12 system were investigated, including its microstructure and infrared reflection spectroscopy. It was found that this system is entirely composed of two phases, BaTi4O9 and BaPr2Ti4O12, and it is suggested that the lattices of each phase are well matched obliquely at the interface in the sintered ceramics. The dielectric properties are compared with calculated values deduced from the mixing relations of the two components. Furthermore, the infrared reflection spectra of this system were analysed with the factorized form of the dielectric function, for the purpose of studying the applicability of the reflection analysis technique to this mixed phase system.