The cobalt aluminum silicides Co19Al45Si10-x (x = 0.13) and Co5Al14Si2 were synthesized in liquid aluminum and characterized by single-crystal X-ray diffraction. Co19Al45Si10-x (x = 0.13) crystallizes in the monoclinic space group C2/c with lattice parameters a = 19.991(2) angstrom, b = 19.143(2) angstrom, c = 12.8137(15) angstrom, beta = 123.583(2)degrees. Co5Al14Si2 adopts the orthorhombic space group Pnma with cell parameters a = 13.8948(19) angstrom, b = 23.039(3) angstrom, c = 7.3397(10) angstrom. Both structures are exceptionally complex with the Co2Si2 rhombus being a common building motif. The coordination environments of cobalt atoms resemble those of the transition metals in typical quasi-crystal approximants. Co5Al14Si2 shows oxidation resistance in air up to 1000 degrees C by forming a dense-packed Al2O3 layer on the surface of the crystal.