The O-methylation effect on the carbon-13 nuclear magnetic resonance (¹³C-NMR) chemical shifts of aryl carbons of the ortho-monosubstituted phenols has been investigated. In phenols with nonconjugated substituents (2-11), O-methylation caused an upfield shift by an average of 4.1 ppm for the ortho-methine carbon (C-6), whereas it caused a downfield shift by an average of 1.1 ppm for the substituted ortho-carbon (C-2). This regularity is very useful for the spectral interpretation of some natural products with an ortho-substituted phenol group, such as flavonoids (16-20 and 22). The O-methylation effect of phenols 12-15 with conjugated ortho-substituents on the aryl carbons, especially C-1, C-2 and C-5, differed significantly from that observed in 2-11. On the basis of a limited number of examples, the O-ethylation effect on aromatic carbons seems similar to the O-methylation effect.