Group IV alloys are attracting strong interest for Si-based optoelectronics. The effects of C on the electrical and optical properties, however, are still not well understood, especially for high Ge content. In this report, we describe optical, structural, and compositional measurements of a series of thick, relaxed p-type Ge1-yCy layers on n-type Si (100) substrates. The alloy layers were 0.5 mu m thick and were grown by solid source molecular beam epitaxy at a substrate temperature of 300 degrees C and p-type doped with different B concentrations. X-ray diffraction indicated that the layers were single crystalline and nearly fully relaxed. The optical absorption was measured using a waveguide structure using Fourier transform infrared spectroscopy. The absorption data versus photon energy data fit indicated an indirect band gap, and one sample had a band gap of 774 meV compared to 660 meV for pure Ge. For single-crystalline, relaxed layers, the effect of C was to increase the band gap energy. These measurements show that alloying Ge with C provides a way to vary the optical absorption, which may be useful for device applications.