Dimeric biphenyl liquid crystals with trisiloxane containing spacers were studied. Both the tail lengths and the spacer lengths to a central hexamethyltrisiloxane group were varied. In most cases a SmC-phase was found with an anticlinic interlayer relation. One of the compounds appears to have a SmA-phase, which could be a de Vries SmA-phase. A few other compounds show liquid crystalline polymorphism. X-ray diffraction experiments show that the smectic layers are strongly interdigitated. The thickness of the layers is mainly dependent oil the length of the spacers. The length of the terminal alkyl chains hardly influences the smectic layer thickness.