Reaction of methylmagnesium iodide with three isomers of methoxymandelonitrile affords methoxyphenylacetylcarbinol (I) and methoxyphenylmethylcarbinol (II) in all cases. The total yield of (I) and (II) are approximately the same in all three cases but the ratio of (I) and (II) differs in each case. From such a result, the formation of (I) and (II) was assumed to be a competitive reaction. By the comparison of the result from the present series of experiments and the reaction of mandelonitrile and methylmagnesium iodide, it was found that the -M (-E) effect of the methoxyl group in the aromatic ring of cyanohydrin, used as the starting material, was disadvantageous to the formation of (I) and advantageous for the formation of (II), while its +I effect had the reverse effect.