While remote detection of oil spills is well-studied, very little success has been reported in estimation of thickness and aging of oil slicks. In this case study, spectral signatures (350 nm - 2400 nm) of oil samples of 5 different thicknesses on saltwater were measured regularly for 2 months. Changes in spectral behaviors were observed over time. Based on our observation, the spectrum of fresh oil surface was initially different from that of saltwater but eventually came to resemble the saltwater spectrum after volatilization and precipitation. Spectra of thinner samples changed more rapidly over time than those of thicker samples. Spectral unmixing was studied of the thin oil samples using thick oil and pure saltwater as endmembers.