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Carboxymethyl kappa-carrageenan (CMKC), is the result of carboxymethylation of kappa-carrageenan. Carboxymethylation of k-carrageenan is carried out in two stages: (1) Alkalization of k-carrageenan using NaOH, to form alkoxy-k-carrageenan, then etherification of alkoxy-k-carrageenan with monochloroacetic acid (MCA) as reactant to produce carboxymethyl k-carrageenan (CMKC). The success of CMKC synthesis can be measured based on the value of the degree of substitution, (DS), DS is the average number of hydroxyl groups that are converted to carboxyl groups. The success of the conversion of hydroxyl groups into carboxyl groups is influenced by several factors, synthesis temperature, types of reactants and the polarity of the reaction medium. This study has been done to analyze the effect of the polarity of the CMKC synthesis reaction medium,to the DS value. k-carrageenan was reacted with NaOH, then the results were reacted with monochloroacetic acid, insome variation proportion of isopropanol-ethanol-acetone, as reaction medium. The value of DS of CMKC that were synthesized was measured by the titrimetric method. The results showed that the polarity of the reaction medium affected the value of DS CMKC. The highest DS value (1,7) was owned by CMKC which was synthesized in the isopropanol-acetone-ethanol (25:3:2) v/v as reaction medium.