We compared hydrology and porewater chemistry along transects in 3 tidal marshes vegetated by Spartina alterniflora and by the invasive species Phragmites australis. Relative to the Phragmites zone, the S. alterniflora zone occurred at lower tidal elevations in all 3 marshes and was characterized by greater depth of flooding and shorter periods of water-table drawdown below the soil surface. Penetration by Phragmites into the S. alterniflora zone appeared to be limited by extensive soil saturation at the leading edge of Phragmites growth. This mixed-species zone occurred at the intersection of the low-tide groundwater table with the soil surface and was characterized by significantly higher concentrations of porewater sulfide (739 ± 63 µ M) relative to the monospecific Phragmites zone (335 ± 63 µ M). Dissolved ammonium and phosphate concentrations were also significantly lower in the Phragmites zone relative to S. alterniflora and mixed-species zones of vegetation. Discriminant analysis of the measured hydrologic and chemical variables identified depth of flooding, flooding frequency and duration, and sulfide concentration, as the most significant variables distinguishing Phragmitesstands along the estuarine salinity gradient. Together, these results indicate a greater flooding tolerance by Phragmites in marshes where porewater sulfide and salinity are lower. From a hydrologic and chemical perspective, opportunities for Phragmites invasion and expansion should be greater in oligohaline and mesohaline tidal marshes relative to polyhaline habitats. To control the growth of Phragmites in tidal marshes of management concern, both the feasibility and need for methods that increase flooding depth, frequency, salinity and/or sulfide concentrations should be considered.