Dielectric and electro-optic properties of two homologues of 4-chlorophenyl 4'-n- alkoxythiobenzoates, showing nematic and smectic A phases with positive dielectric anisotropy, were studied. Planar alignment of the nematic was obtained upon slowly cooling from the isotropic phase using ITO planar cell. Upon further slowly cooling to the smectic phase one can obtain its planar texture. Reversible Frederiks transition was observed for nematic phase in AC field. It is possible to measure the splay elastic constant which changes discontinuously at the nematic-smectic A transition. However, in the smectic phase Frederiks transition was irreversible and the threshold voltage was much higher.