Thin films of crystalline lanthana and La-Mn oxide prepared on p-Si substrates were characterised by energy dispersion X-ray fluorescence (XRF) and X-ray diffraction (XRD) methods. The XRF spectrum was used to determine, the weight fraction ratio of Mn to La in the prepared samples. Crystalline hydroxide compounds (La(OH)3 and LaOOH) in addition to La2O3 structure were detected in the prepared oxidised lanthanum films pre-annealed at 600 degrees C. The XRD of La-Mn oxide films shows that La oxide and Mn oxide do not prevent each other to crystallise each alone at 600 degrees C and they do not form a solid solution or a complex compound. The lanthanum hydroxide compounds were not detected in La-Mn oxide samples. The hydroxide compounds reduced the dielectric constant of the prepared lanthana. The ac-conductance and capacitance were studied, as a function of frequency and gate voltage and the fixed charge and interface state densities were determined. It was found that the "correlated barrier hopping" CBH model controls the frequency dependence of the conductivity, while the Kramers-Kronig (KK) relations explain the frequency dependence of the relative permittivity. The parameters of CBH model were determined showing that the ac-conduction is realised by a single-polaron hopping mechanism. Data show that La-Mn oxide and La oxide films stored in normal ambient have almost similar ac properties. (c) 2007 Elsevier B.V. All rights reserved.