New organic rare-gas compounds, HRgC(4)H (Rg = Kr or Xe), are identified in matrix-isolation experiments supported by ab initio calculations. These compounds are the largest molecules among the known rare-gas hydrides. They are prepared in low-temperature rare-gas matrixes via UV photolysis of diacetylene and subsequent thermal mobilization of H atoms at similar to30 and 45 K for Kr and Xe, respectively. The strongest IR absorption bands of the HRgC(4)H molecules are the H-Rg stretches with the most intense components at 1290 cm(-1) for HKrC4H and at 1532 cm(-1) for HXeC4H, and a number of weaker absorptions (C-H stretching, C-C-C bending, and C-C-H bending modes) are also found in agreement with the theoretical predictions. As probably the most important result, the IR absorption spectra indicate some further stabilization of the HRgC(4)H molecules as compared with the corresponding HRgC(2)H species identified recently (Khriachtchev et al. J. Am. Chem. Soc. 2003, 125, 4696 and Khriachtchev et al. J. Am. Chem. Soc. 2003, 125, 6876). The computational energetic results support this trend. HXeC4H is predicted to be 2.5 eV lower in energy than H + Xe + C4H, which is similar to1 eV larger than the corresponding value for HXeC2H. We expect that the larger molecules HRgC(6)H and HRgC(8)H are even more stable and the HRgC(2n)H species are good candidates for bulk organic rare-gas material.