In order to distinguish aged paint media by Fourier transform infrared (FTIR) spectroscopy, the authors focus on: 1) the natural aging of well-defined macroscopic samples of paint media; 2) the modifying effects of typical artists' pigments on the aging of these media; and 3) the utility of these findings in the characterization of unknown media samples. The key findings are that samples of linseed oil, egg yolk, and an emulsion of linseed oil and egg yolk, prepared by R.J. Gettens in the 1930s, exhibit diagnostic IR spectral features well resolved from common inorganic pigment absorptions, thus enabling us to distinguish between these materials and also to detect their presence in authentic Renaissance paintings. Moreover, thermally accelerated aging of freshly prepared linseed oil films yields FTIR spectral features that agree closely with those obtained for the naturally aged Gettens samples of similar initial composition. The results further demonstrate that the chemical changes associated with either natural or thermally accelerated aging of oil films vary widely with the choice of the inorganic pigment present. Results presented here serve to provide a spectroscopic characterization of the drastic, pigment-dependent chemical changes that have previously hampered the identification of linseed oil on aged paint samples. Successful extension of these results to the spectroscopic analysis of dimensionally resolved layers of paint will be addressed in subsequent publications.