Using high performance liquid chromatography (HPLC) with a reverse phase column and a photodiode array detector, the chromatographic profiles of the four red onion varieties 'Mambo', 'Red Bone', 'Red Granex' and 'Red Jumbo' were determined. A total of four major and four minor anthocyanins were present in red onions. The anthocyanins and colourless phenolics were identified using a rapid identification procedure based on spectrophotometric, and liquid and gas chromatographic techniques. The major anthocyanins were identified as cyanidin 3-glucoside, cyanidin 3-laminaribioside, cyanidin 3-(6$\sp{\prime\prime}$-malonylglucoside), and cyanidin 3-(6$\sp{\prime\prime}$-malonyllaminaribioside). The four minor anthocyanins were identified as cyanidin 3-(3$\sp{\prime\prime}$-malonylglucoside), peonidin 3-glucoside, pe nidin 3-malonylglucoside and cyanidin 3-dimalonyllaminaribioside. Using the same technique, the colourless phenolic compounds were identified as protocatechuic acid 4-glucoside, quercetin 7,4$\sp\prime$-diglucoside, quercetin 3,4$\sp\prime$-diglucoside, quercetin 3-glucoside, quercetin 4$\sp\prime$-glucoside, and a 5,7-dihydroxy flavanone glucoside or a 5,7-dihydroxy dihydroflavanol glucoside. The copigmentation effect was studied using cyanidin 3-glucoside, cyanidin 3-malonylglucoside, cyanidin 3-malonyllaminaribioside, chlorogenic acid, protocatechuic acid and caffeine in aqueous buffers at pH 3.7, 4.7 and 5.7. It was observed that copigment structure and concentration, and pH have a dramatic influence on the copigmentation phenomenon. Colour stability studies using cyanidin 3-glucoside solutions at pH 3.7, 4.7, and 5.7 without and with added chlorogenic acid, protocatechuic acid and caffeine in the presence and absence of light, demonstrated that intense exposure to fluorescent and incandescent light was detrimental to the photostability of copigmented cyanidin 3-glucoside solutions at any pH. (Abstract shortened by UMI.)