The UV and polarized IR spectra of argon matrix isolated cyclobutadiene and its 1-d-, 1,2-d2-,1,4-dr, 1-C-13-, 1,2-C-13(2)-, and 1,4-C-13(2)-labeled derivatives are reported. Above 250 nm, the UV spectrum contains no peaks with an extinction coefficient larger than 10 L M-1 cm-1, but a weak absorption tail extends throughout the UV region. The absorption rises abruptly below 250 nm. The IR peak positions mostly agree with previous reports where these are available. Photoalignment studies support the symmetry assignment of fundamental vibrations and prove that the two Kekule forms interconvert rapidly even at 10K. The spectral and photochemical effects of the presence of an adjacent CO2 molecule in the cyclobutadiene matrix site are elucidated. Its presence causes a substantial retardation of the photofragmentation of cyclobutadiene into two acetylenes. This is attributed to accelerated vibrational energy loss from the cyclobutadiene absorber.