The vibrational signatures for the v(2) C (math)C and v(1) symmetric C-H stretches of acetylene in cubic structure I clathrate, synthesized under ambient pressure, are reported for the first time. The most diagnostic features are at 1966 for v(2) and 3353 cm(-1) for v(1), respectively, and are assigned to acetylene trapped in the large 5(12)6(2) cages. In addition, the v(2) mode for acetylene occupying the small 5(12) cages is observed at 1972.5 cm(-1), a red shift of 1.5 cm(-1) from its gas phase frequency. Unit cell parameters and thermal expansion coefficients are determined via powder X-ray diffraction between 195 and 225 K and are found to be in good correlation with previous single crystal data at 143 K. The calculated density for acetylene clathrate is also reported, with values ranging from 0.985 g/cm(3) at 195 K to 0.976 g/cm(3) at 225 K. These results are relevant for spectral detection of acetylene-containing compounds on planetary bodies, as well as providing additional insights on the thermal behavior and physical properties of acetylene clathrate.