In this study, electron spin resonance (ESR) spectroscopy was used for the first time to investigate liquid diffusion into contact lenses. As contact lenses are not paramagnetic substances, they were labeled with nitroxide spin probes to get an ESR spectrum, Thus, it gives a solid spinlabeled ESR spectrum. The shape and intensity of the ESR signals depend on the environment of these spin probes. The spin probe environment began to change from solid to liquid if liquid were dropped into the system. Consequently the ESR spectra began to change with time, too. By following these changes, three distinct steps were found. Their diffusion coefficients were determined to be 6.38 × 10 -8 cm2/s for the first step (rapid decay region) and 0.37 × 10-8 cm2/s for the second step (slow decay region), and 2.50 × 10-8 cm2/s for the third and last step (desorption region). © 2006 Wiley Periodicals, Inc.