Dielectric　Properties　of　bismuth　layer　type　ceramics　with　alkali　and　lanthanum　substitutions　(Pb1-x,　Mx/2,　Lax/2)　Bi2Nb2O9　are　presented.　Dielectric　constant　and　loss　tangent　of　the　substituted　materials　at　room　temperature　are　all　higher　than　the　parent　PBN.　At　temperatures　higher　than　T0,　dielectric　constants　deviate　from　Curie-Weiss　law　showing　relaxor　behavior.　The　new　materials　exhibit　very　high　resistivity　and　break-down　strength　at　high　temperatures　making　these　materials　very　attractive　for　capacitor　dielectrics.　©　1985　The　Japan　Society　of　Applied　Physics.