Citric acid (CA), one of the polycarboxylic acids used as cross-linking agents for cotton, is cost-effective and environmentally friendly, but its tendency to discolor fabric is a major disadvantage for white fabrics. In this research, we investigate cotton fabric yellowing caused by citric acid at elevated temperatures. We find that yellowing increased as curing temperature, curing time, and CA concentration increase. Using sodium hypophosphite as a catalyst causes less fabric yellowing than monosodium phosphate. The wet analysis data demonstrate that an alkene double bond is formed on cotton fabric treated with CA under curing conditions. Nuclear magnetic resonance (NMR) data indicate that CA is partially converted to trans- and cis-aconitic acids on cotton through dehydration. The visible spectra of fabric treated with trans- and cis-aconitic acids and cured at elevated temperatures show strong absorption in the 400-550 nm region. For polymeric multifunctional carboxylic acids, an increased number of hydroxyl groups in their molecules increases fabric yellowing. Therefore, yellowing caused by citric acid and other hydroxyl multifunctional carboxylic acids can probably be attributed to the formation of unsaturated polycarboxylic acids.