A systematic relationship between the molecular structure of methoxy-substituted benzenediazonium salts and the N-N stretching frequency has been observed. This is useful in the identification and classification of these molecules. Resultant substituent constants, σ+, for multiple substituted derivatives were assigned on the basis of their observed N-N frequencies and a Hammett ρ-σ- correlation of the rate constants for the coupling reaction with 6-naphthol-2-sulfonate produced ap of 3.05. © 1965, American Chemical Society. All rights reserved.