<![CDATA[Abstract The use of synthetic dyes in the textile industry can negatively affect the environment because they generate chemical waste that is difficult to degrade. One alternative for developing sustainable textiles is the use of natural dyes derived from plants, including ketapang leaves (Terminalia catappa), which contain tannins and flavonoids as natural coloring compounds. However, natural dyes have limitations in color stability and colorfastness, making the use of biomordants necessary to improve dyeing quality. This study aimed to determine the effects of organic acid biomordants—vinegar (acetic acid), citric acid, and lactic acid—on the color characteristics and colorfastness of ketapang-leaf natural dye on Primisima cotton fabric. This study employed an experimental method with four treatments: no biomordant, vinegar, citric acid, and lactic acid. Color characteristics were assessed by 15 panelists, all of whom met the color-perception test criteria. Colorfastness was tested according to SNI ISO 105-C06:2020, with color change evaluated using SNI ISO 105-A02:2010 and staining evaluated using SNI ISO 105-A03:2019. The data were analyzed using the Friedman test to identify differences among treatments. The results showed that organic acid biomordants significantly affected color evenness, according to the Friedman test (p = 0.016, <0.05). The mean color-evenness score was 3.2667 for the treatment without a biomordant and 4.0000 for vinegar, citric acid, and lactic acid. In terms of color characteristics, lactic acid produced predominantly Clam Shell Pink (80%), while citric acid produced Dark Salmon Pink (60%). Colorfastness testing yielded color-change ratings of 3–4 for the untreated, citric-acid, and lactic-acid samples, while vinegar received a rating of 3. All treatments achieved staining ratings of 4 on cotton adjacent fabric and 4–5 on polyester adjacent fabric. These findings indicate that organic acid biomordants can help improve color stability and support the quality of ketapang-leaf natural dye on Primisima cotton. Keywords: ketapang leaves, natural dye, biomordant, organic acids, Primisima cotton fabric, colorfastness]]>
