Contributions of organic alkalinity (Org-Alk) to total alkalinity (TA) were investigated in surface waters from three different coastal environments (estuary, urban, mangrove) and offshore sites in the Gulf of Mexico. ∆ TA was calculated as the difference between directly measured TA, and TA calculated from total dissolved inorganic carbon (DIC) and pH. In low nutrient surface waters, ∆ TA should be dominated by Org-Alk with minor contributions from inorganic nutrients (e.g., HPO₄^2 − and SiO(OH)₃⁻). Average values of ∆ TA were 0.1 ± 5.0 μmol kg^− 1 at coastal sites outside the Mississippi–Atchafalaya River Estuary (n = 17), 33.6 ± 18.0 μmol kg^− 1 in the Suwannee River Estuary (n = 17), 16.0 ± 25.4 μmol kg^− 1 in the Tampa Bay, Caloosahatchee River, and Ten Thousand Islands area (n = 55), and − 1.0 ± 4.9 μmol kg^− 1 in offshore waters (n = 14) in the northern Gulf of Mexico. In addition to Org-Alk assessments based on ∆ TA, procedures were developed for direct spectrophotometric measurements of Org-Alk via titrations of samples that were purged of CO₂. Two-step titrations of these DIC-free samples consisted of a first titration from pH 4.5 to 6.0 performed using bromocresol purple (BCP), and a second titration, from pH 6.0 to about 8, using cresol red (CR) as the indicator. By diluting all samples, including the offshore reference sample, to a common salinity (the lowest salinity of the coastal samples), borate alkalinity was presumed to be identical for all samples. Org-Alk values were calculated as differences between titration results obtained for coastal samples and the offshore reference sample and, through ancillary nutrient measurements, accounted for alkalinity contributions from silicate and phosphate. The direct titrations confirmed the existence of substantial Org-Alk in coastal samples. Spectrophotometric titration data were also used for model fitting in order to assess the dissociation constants (pK_i) of the organic acids. The pK_i of the organic acids were within the previously reported range for riverine fulvic acids.