The syntheses of several 4-n-alkyl-, 4-[2-(trans-4-n-alkylcyclohexyl)ethyl]-, 4-[2-(4-n-alkylphenyl)ethyl]- and 4-n-alkoxy-4′-cyano-2′-fluorobiphenyls and -4′-cyano-2-fluorobiphenyls are presented and the transition temperatures for these compounds are reported and discussed. In the alkylcyanobiphenyls, a 2-fluoro-substituent causes a greater depression of TN-1(35.39[ddot]C) than a 2′-fluoro-substituent (13-18[ddot]C) and a similar situation arises for the cyclohexylethyl compounds. The fluoro-substituted alkoxy-compounds also give two series with depressions of TN-1of 34-41[ddot]C for the 2-fluoro and 15-20[ddot]C for the 2′-fluoro-compounds. In non-cyano systems, 2- and 2′-fluoro-substituents in biphenyl derivatives give similar depressions in TN-1, which are larger than those found in cyano systems, and the different depressions for the fluoro-substituted cyano compounds are thought to arise because of the effect of the fluorine on the anti-parallel correlations of the parent systems. The effects on smectic and nematic thermal stabilitis are qualitatively similar, but are usually greater for smectic transitions.