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Abstract. Carboxymethyl k-carrageenan (CMKC), a derivate product of kappa carrageenan, is one of coating material that is used for nanomagnetite coated production. The important parameter to determining quality of CMKC is degree of substitution (DS). DS value indicated the number of hydroxyl groups that were converted into carboxyl groups. Polarity of synthesis reaction medium affect to the DS value of CMKC that was synthesized. This research was arranged to analyze the effect of synthesis reaction medium polarity on the DS value of CMKC, we was studied the use of isopropanol-ethanol-ethyl acetate mixture at some proportion variation, as synthesis reaction medium, relation between polarity of reaction medium and DS value of CMKC, then determine optimum proportion of isopropanol-ethanol-ethyl acetate, asreaction medium, that was produced CMKC that has highest DS value. This research was done in two step (1) synthesize of cmkc by monochloroacetic acid (MCA) as etherification agent and mixture of isopropanol-ethanol-ethyl acetate as reaction medium (2) characterization of CMKC properties to determine the effect of synthesis reaction medium polarity against DS value. The results showed that reaction medium polarity affect on the value of DS. CMKC synthesis that was done in isopropanol-ethanol-etyl acetate 25:2:3 v/v has the highest DS value, 1.60.