The Kl-xBaxNbO3 (0.2 less than or equal to x less than or equal to 0.5), BaNbO3 and K0.5Sr0.5NbO3 compounds have been synthesized by high-pressure high-temperature technique in a Belt-type apparatus. The synthesis of the K-containing phases carried out in evacuated sealed silica tubes led to the preparation of polyphasic samples. The phases obtained have an ideal perovskite-like structure. The a parameter of Ba-containing compounds increases concomitantly with increasing Ba concentration. Studies of the K-containing phases by X-ray powder diffraction did not reveal any distortion from cubic symmetry. Electron Dispersive Spectroscopy (EDS)-analysis confirmed the cation nominal stoichiometry. Resistivity me asurements showed semiconducting-like type of conductivity for the Kl-xBaxNbO3 (0.2 less than or equal to x less than or equal to 0.5) and K0.5Sr0.5NbO3 samples, but the relative increase of rho with the decrease of temperature became smaller for the phases with lower Nb valence, and moreover K0.5Sr0.5NbO3 exhibited a slight decrease of the resistivity in the 100-4.2 K temperature range. On the contrary, BaNbO3 after high-pressure treatment showed a metallic-like behaviour down to 4.2 K.