The C-H insertion reaction of laser-ablated Hf atoms with ethylene in excess argon has been carried out during codeposition at 7 K, and the reaction products have been investigated by means of infrared spectroscopy. Parallel to the case of Zr, mono-, di-, and trihydrido C-H insertion complexes are identified. Among them, the mono- and dihydrido C-H insertion complexes are reaction intermediates of hydrogen elimination from ethylene by second-row transition-metal atoms presumed in previous reaction dynamics studies. The higher Hf-H stretching frequencies and shorter Hf-H and C-Hf bond lengths of the hafnium C-H insertion products than those of the zirconium products are attributed to the shortened bonds of the heavier metal atom by relativistic effects.