A time resolved optoacoustic technique was used to measure the vibrational relaxation times of SiHnF4-n (n=4, 3, 2, and 0) diluted in Ar. Fundamental vibration-rotation bands of these molecules were excited by using a line tunable CO2 laser. It was found that several vibration-rotation lines of SiH4(v4), SiH3F(v4), SiH2F2(v2) and SiF4(v3) coincided with the CO2 laser lines, as shown in Fig. 1. The vibrational relaxation times obtained in the present work are summarized in Table 1 together with the existing experimental data on CH4 and its fluorine substituents. It is noted that the self-relaxation rate of SiH2F2 is slower than that of CH2F2 in spite of its lower frequency of the lowest vibrational normal mode. This observation is in conflict with the prediction of simple adiabatic theories for V-T energy transfer. A theory for V-R, T energy transfer proposed by Nikitin and Moore was applied to explain this anomaly. Predictions of this V-R, T theory were found to be consistent with experimentally observed variation of the relaxation rates along with the number of hydrogen atoms, n, in SiHnF4-n and CHnF1-n. Slower vibrational relaxation of SiH2F2 than that of CH2F2 is caused by its larger effective reduced mass which is defined by equation (4). Not only the amount of energy transfered but also the effective reduced mass have large effect on the vibrational relaxation rates of these molecules.