Near-field scanning microwave microscopy was applied to investigate the dielectric properties and microstructure in a polycrystalline LaAlO3-TiO2 diffusion couple, which included three regions containing different phases and microstructures. Relatively low (La2Ti4Al18O38), high (alpha-La2/3TiO3), and intermediate (La4Ti9O24) dielectric constant phases were distinguished at the inter-diffusion interface in optical, backscattered electron scanning electron microscopy, and scanning microwave microscopy (SMM) images. The relative ranking of dielectric constants based on SMM examination was as follows: TiO2 >alpha-La2/3TiO3 > La4Ti9O24 > LaAlO3 > La2Ti4Al18O38. La2/3TiO3 and LaAlO3 will form solid solutions in the LaAlO3-rich region. The reaction paths leading to phase development are discussed.