The synthesis and structure of the thiogermanic acids H4Ge4S10 and H2Ge4S9 are reported. A novel preparation method consisting of reacting germanium oxide with liquid hydrogen sulfide containing a trace amount of water is used to form Ge4S104- ions. Evaporating the hydrogen sulfide solution at room temperature leaves an unstable H4Ge4S10.xH(2)O product. The stoichiometry and structure of the thermally stable anhydrous phase are dependent on reaction time. An H4Ge4S10 product with an adamantane-like cage structure is obtained at shorter reaction times. Longer reaction times produce an H2Ge4S9 product with a more complex cage unit, a higher symmetry unit cell, and increased thermal stability. Raman, infrared, powder X-ray diffraction, and thermogravimetric data are reported for both structures.