The total phenolic and vitamin C contents of organic table grapes were determined by spectrophotometry and HPLC, respectively, after several quality-increasing treatments were applied at veraison for two growing seasons. Cane girdling and cluster-berry thinning resulted in different responses in the cultivars. 'Red Globe' generally was found to produce the lowest quantity of total phenolics in the berries, while 'Trakya Ilkeren' was the cultivar that accumulated the highest phenolics. Vitamin C accumulation in the berries was also affected by the treatments. The effect of the treatments was inconclusive in 'Buca Razakisi'. The application of both girdling and thinning caused more accumulation of vitamin C in the 'Alphonse L.' and Trakya Ilkeren' berries.