Absolute infrared absorption spectra of the linear (hexamethyldisiloxane ((CH3 )3 SiOSi(CH3 )3 , L2 ), octamethyltrisiloxane ([(CH3 )3 SiO]2 Si(CH3 )2 , L3 ), decamethyltetrasiloxane ([(CH3 )3 SiO]2 (Si(CH3 )2 OSi(CH3 )2 ), L4 ), dodecamethylpentasiloxane ([(CH3 )3 SiO]2 (Si(CH3 )2 OSi(CH3 )2 OSi(CH3 )2 ), L5 ), and cyclic hexamethylcyclotrisiloxane ([–Si(CH3 )2 O–]3 , D3 ), octamethylcyclotetrasiloxane ([–Si(CH3 )2 O–]4 , D4 ), decamethylcyclopentasiloxane ([–Si(CH3 )2 O–]5 , D5 ), and dodecamethylcyclohexasiloxane ([–Si(CH3 )2 O–]6 , D6 )) permethylsiloxanes were measured over the 60 0–40 0 0 cm−1 spectral region at 294 K using Fourier transform infrared (FTIR) spectroscopy at 1 cm−1 resolution. Measurements were performed using manometrically prepared dilute permethylsiloxane mixtures in a He bath gas and infusion of the pure compound into a calibrated gas flow. Theoretical calculations at the B97-1/6-311++G(2df,2p) level of theory were performed and optimized permethylsiloxane geometries and vibrational band positions and strengths are reported. The theoretically calculated infrared spectra are compared with the experimental spectra and found to be in good agreement. The radiative efficiencies (RE) of the permethylsiloxanes were calculated and found to be extremely large in comparison with other atmospheric greenhouse gases, with values ranging from 0.55 to 2.02 W m−2 ppb−1 for a uniform atmospheric distribution, i.e., well-mixed scenario.