Mixed Si/Ge hydrides SixGeyHz are valuable precursors for the deposition of binary Si-Ge alloys. This work describes the synthesis and full characterization of the previously unknown germaisotetrasilane Ph3GeSi(SiH3)(3) (2) on a multigram scale from the reaction of the lithium silanide LiSi(SiH3)(3) with Ph3GeCl. The stability of the Si-Ge bond in 2 versus electrophiles and nucleophiles has been investigated with the primary aim of developing new approaches to mixed sila-H-germanes (H3Ge)(x)Si(SiH3)(4-x). With 1 equiv of MeLi, 2 reacted cleanly under cleavage of one Si-Si bond to give Ph3GeSi(SiH3)(2)Li, which is a valuable synthon for further derivatization. In contrast, the dephenylation reaction of 2 with 1 or 2 equiv of CF3SO3H/iBu(2)AlH proceeded much less selectively and afforded the desired Ph/H-germasilanes Ph2HGeSi-(SiH3)(3) and PhH2GeSi(SiH3)(3) along with considerable amounts of Si-Ge scission products.