The dielectric properties of (Ba0.5Sr0.5)TiO3 (BST) thin films with high electrical resistivity were investigated. BST films are deposited on Pt/TiO2/SiO2/Si substrates by a metal-organic deposition (MOD) method. The dielectric permittivity and ac conductivity of the films are measured in the frequency range 102-105 Hz. The dielectric permittivity epsilon (r) decreases slightly with frequency f, following the relationship epsilon (r) = a + bf(n-1) (a, b and n are constants, n < 1). The ac conductivity (f) increases with frequency as sigma>(*) over bar * (f) similar to f(n). These results indicate that the phonon-assisted jumps of electrons between localized states play an important role in the dielectric properties of BST thin films. The dielectric permittivity and ac conductivity of the BST thin films increase with grain size, and decrease with increasing temperature. A preliminary explanation is given.