Reentrant nematic phases and smectic antiphases occur as intermediate states between two one-dimensionally ordered frustrated smectic phases, characterized by two competing and, in general, incommensurate smectic density waves. Using high-resolution x-ray diffraction, we have examined the evolution of the two density waves as a function of temperature in all phases of a mixture of polar liquid crystals octyl- and decyl-oxyphenyl nitrobenzoyloxy benzoate (47.4 mol% DB8ONO2+52.6% DB10ONO2). This mixture exhibits at temperatures below 127 °C the liquid-crystal phase sequence: smectic-Ad, reentrant nematic phases Nd and N1, smectic-A1, tilted antiphase C̃, smectic-A2, and smectic-C2 phases. Detailed investigations of several order-disorder transitions and the evolution of fluctuation effects in this system are reported