Single crystals of two mixed-valence lanthanum titanium(III/IV) oxosilicates, alpha- and beta-La4Ti9Si4O30, were grown by a high-temperature, solid-state reaction employing BaCl2 molten salt. Their structures were determined by single crystal X-ray diffraction methods. Both phases crystallize in a monoclinic lattice, C2/m (No. 12), with a = 13.545(2) Angstrom, b = 5.571(1) Angstrom, c = 15.1888(9) Angstrom, beta = 110.922(7)degrees, and V = 1105.1(3) Angstrom(3) for alpha-La4Ti9Si4O30 and a = 13.536(2) Angstrom, b = 5.750(1) Angstrom, c = 14.252(1) Angstrom, beta = 95.387(8)degrees, and V = 1104.4(3) Angstrom(3) for beta-La4Ti9Si4O30 with Z = 2. These phases are polymorphs which represent the second member (m = 2) of the layered oxosilicate series, La4Ti(Si2O7)(2)(TiO2)(4m). The two-dimensional framework can be viewed as a rutile lattice, TiO2, that is sliced by closed-shell, nonmagnetic silicate slabs, La4Ti(Si2O7)(2), along the (110) plane at various octahedra thicknesses, m. The physical and electronic properties of this layered compound series were characterized by measuring their electrical resistivities and calculating their electronic band structures with the extended Huckel tight-binding (EHTB) method. The bulk resistivity showed semiconducting behavior with small gap energies. When the temperature is lowered, the m = 1 and 2 phases exhibit a sharp resistivity increase below similar to 20 and similar to 80 K, respectively.