Abstract <p>Based on the nucleophilic addition reaction between cinnamaldehyde and bisulfite, this study develops an analytical method for determining the cinnamaldehyde content in cinnamon essential oil using iodometric back-titration. Cinnamaldehyde reacts quantitatively with excess bisulfite, and the unreacted bisulfite is titrated with iodine. The method demonstrated a strong linear relationship (<i>R</i>² = 0.9999) within the concentration range of 2–15 mg/mL, with a detection limit of 0.32 mg/mL and a quantification limit of 1.07 mg/mL. Recovery tests using Chinese and Ceylon cinnamon essential oils showed recovery rates between 96.0 and 99.4%, with intra-day and inter-day relative standard deviations below 2%. The results were consistent with those obtained by gas chromatography. The method is accurate, reproducible, and cost-effective, and does not require sophisticated instrumentation, making it widely applicable for quality control and authenticity assessment of cinnamon essential oils. This work provides a low-cost and reliable method for the determination of cinnamaldehyde in cinnamon essential oil.</p>