This article is the first part of a three-part review of the more popular liquid phases used in preparing packed columns for gas chromatographic use. The purpose is to present information concerning structure, chromatographic selectivity, and precautions when using these liquid phases. This part discusses the use of polysiloxanes with pendant methyl, phenyl, trifluoropropyl, and cyanoalkyl groups. For general use, polymethylsiloxanes are recommended. Phenylsiloxanes, available with phenyl contents from 5 to 75%, are recommended for general analysis and particularly for chromatographic retention of polarizable groups. The trifluoropropylsiloxanes are recommended when one wishes to achieve unique selectivities of compounds bearing hydroxyl and carbonyl groups. The cyanoalkylsiloxanes, also available in a wide range of cyano content, will retain compounds that have pi-electrons, such as aromatic compounds, and are recommended for the separation of cis- and trans-olefin isomers. All siloxane polymers are susceptible to air oxidation, especially at temperatures above 150°C. They should be protected from oxygen and water by placing suitable traps in the carrier gas lines. Siloxane liquid phases will degrade if Lewis acids or bases, strong acids, and, in particular, strong bases are injected onto them at high temperatures.