A new method for in-situ detection and measurement of dissolved methane in aqueous media/environments with a limit of detection of 0.2 nM (3 sigma, and t90 approxiamtely 110s) and range (1-300 nM) is presented. The detection method is based on refractive index (RI) modulation of a modified PolyDiMethylSiloxane (PDMS) layer incorporating molecules of cryptophane-A  which have a selective and reversible affinity for methane . The refractive index is accurately determined using surface plasmon resonance (SPR) . A prototype sensor has been repeatedly tested, using a dissolved gas calibration system under a range of temperature and salinity regimes. Laboratory-based results show that the technique is specific, sensitive, and reversible. The method is suitable for miniaturization and incorporation into in situ sensor technology.