The surface tension-temperature characteristic of the homologous series 4,4′-di-n-alkanoyloxy-biphenyls (BP-n: CnH2n+1CO2-C6H4-C6H4-CO2CnH2n+1) has been investigated by using the computer-aided pendant drop method. In casen> 3 BP-n's are characterized by a highly ordered smectic G phase directly below the isotropic phase. Positive temperature coefficients of surface tension, which implies a surface-molecular excess order, are generally observed as the temperature is lowered to approach the phase transition. The remarkably wide temperature range of this anomaly is consistent with the suggestion that the existence of an underlying highly ordered phase favors the pretransitional surface ordering at the free surface of an isotropic phase.