An assessment using ERS-1 SAR data to differentiate methane source (wetland) and non-source (non-wetland) areas was undertaken based on radar backscatter modeling and empirical observations of 24 scenes collected over Barrow, Alaska in 1991 and 1992. Differences in backscatter between source and non-source areas were dependent on surface hydrology and air temperature. Differential freezing of surface materials on daily to seasonal time scales greatly enhanced the separability of wetlands and non-wetlands with ERS-1 SAR. Radar return for non-wetlands decreased dramatically whereas backscatter from wetlands decreased little when freezing air temperatures coincided with the SAR overpass. Maximum separability between wetlands and non-wetlands, as determined from observed and modeled radar backscatter, were the result of changes in the dielectric constant of the plant and surface materials with phase change during freezing. This study has indicated the need to consider air temperature at the time of acquisition in selecting ERS-1 SAR scenes for differentiating methane source and non-source areas.