Northeast monsoon plays a vital role in the regulation of physico-chemical characteristics in the Edaiyur -Sadras estuarine system as well as in the adjoining coastal waters. The Edaiyur and Sadras Backwaters get connected to the sea during the NE monsoon period resulting in an estuarine condition in the coastal waters. Surface temperature of these backwaters and coastal waters are characterised by maxima in April and in October and minima in January and duly. Fresh water discharge during NE monsoon period from the backwaters causes considerable dilution of coastal waters resulting in a marked reduction in surface salinity (22 ppt). With the stoppage of monsoon rains a gradual build-up of salinity is found in both the backwaters due to the continuous ingress of sea water. The DO in the coastal waters showed relatively high values (8 ppm) during the monsoon period due to the heavy inflow of fresh water (which is high in DO) from the backwaters. The secchi disc depth in the coastal water decreased substantially (7 m to 0.75 m) during the NE monsoon period due to the heavy runoff of turbid fresh water from the backwaters. The plant nutrient values in the coastal waters also increased during the NE monsoon period following the opening of the sand bar which separates the backwaters from the sea during other periods. The Sadras Backwater is highly rich in nutrients due to the presence of a sewage outfall located upstream in this backwater.