Variations in hydrologic input and tides control exchange of water in estuarine systems. Understanding hydrologic mixing in estuaries is important for modeling the transport, fate and cycling of nutrients and contaminants from inland sources. Although several studies have examined estuarine hydrologic mixing, the timing of sample collection relative to tidal stage has not been investigated. The objective of this study was to evaluate salinity, dissolved inorganic carbon (DIC), alkalinity and the isotopic ratio of DIC in assessing estuarine hydrologic mixing during low, high and rising tides. This study was conducted in the Blackwater River estuary in southwest Florida, USA, during the dry season in February 2001. Salinity, DIC, alkalinity and the isotopic ratio of DIC varied from 34.9 to 38.9 ppt, 27.7 to 65.7 mg C/l, 126 to 279 mg/L and –2.1 to –11.4 per mil (relative to PDB standard). Spatial trends in salinity showed higher salinity values between the headwater and mouth of the estuary and a gradual decrease in salinity further offshore. This suggests input of water with higher salinity in portions of the estuary. Spatial trends in DIC and alkalinity were similar and decreased from the headwater to the mouth of the estuary for all tidal stages. The isotopic ratio of DIC increased from the headwater to the mouth of the estuary for all tidal stages. In the absence of freshwater input into the estuary, a salinity-carbon isotopic mass balance model was not able to predict mixing of ocean water in the estuary for any of the tidal stages. In contrast, a simple DIC-carbon isotopic regression model proved to be useful in assessing ocean water mixing in the estuary for all tidal stages. The results of this study suggest that tidal stages affect the degree of hydrologic mixing within estuarine systems and should be considered in estuarine mixing models.