We report a new method developed for the isolation and determination of low molecular weight dicarboxylic acids and related polar compounds (C₂−C₉) from seawater samples. The seawater sample was first acidified and then passed through an activated charcoal column to adsorb the dicarboxylic acids. They are then desorbed with NH₄OH/methanol/water mixture and derivatized with BF₃/1-butanol to dibutyl esters, which are determined using a capillary GC/FID. We tested different pH (0.5, 1, 1.5) and different amounts of charcoal (100, 200, 300, 500 mg) to maximize the recovery of dicarboxylic acids. Using 300 mg of activated charcoal for 100 mL of sodium chloride solution acidified at pH 1, the recoveries for C₂, C₃, C₄, C₅, C₆, C₇, C₈, and C₉ were 61, 49, 96, 84, 77, 67, 57, and 53%, respectively. We applied this method to real seawater samples collected from the northwestern Mediterranean Sea. Here, we report, for the first time, concentrations of a homologous series of C₂−C₉ dicarboxylic acids as well as ketocarboxylic acids including glyoxylic and 4-oxobutanoic acids in surface water and deep seawater samples, with glyoxylic acid being the dominant species (8−40 μg L^-1). This method allows the detection of dicarbonyls such as glyoxal in the samples although its quantification is not possible.