Polydimethylsiloxane (PDMS) is an important polymer with numerous applications. Herein, the authors report the optical function(s) of PDMS from 191 to 1688 nm as determined from reflection spectroscopic ellipsometry (SE) and transmission ultraviolet-visible data. Two commercial samples of liquid PDMS (PDMS700 and PDMS2000) with molecular weights of 700–1500 g/mol and 2000–3500 g/mol, respectively, were analyzed. Both samples were linear polymers terminated with silanol groups. The optical functions determined from the two materials were essentially identical. Both the reflection and transmission measurements obtained from these materials required special experimental considerations. For the reflection SE measurements, these included roughening (frosting) the inside of the vessel that held PDMS and the need to level the instrument instead of the samples. The transmission measurements were obtained via a dual cuvette approach that eliminated the effects of the cuvettes and their interfaces. In addition to analyzing the data from the individual samples, the SE data from the samples were considered together in a multisample analysis (MSA). Because both samples of PDMS were transparent over the measured wavelength range, and because of the relatively wide wavelength range considered, the optical functions here were fit with Sellmeier models. This produced a good fit for the MSA with a mean squared error value of 1.68. The optical functions obtained in this work agreed well with previously reported values. For example, for the MSA, the authors obtained the following nx values, where x is the wavelength in nanometers: n300 = 1.443, n500 = 1.407, and n1000 = 1.393.