In applying the Hammet equation the naphthyl derivatives, the fused benzene ring may be regarded as a group attached to a corresponding phenyl derivative, and its influence on reactivity may be discussed in terms of the Hammet substituent constant δ. Such discussion, in the case of a 1-naphthyl derivative is severely limited because the electronic effect (which is measured by δ) is masked by steric effects, the magnitude of which cannot normally be determined. Simple reactions of 2-naphthyl derivatives are free from such steric complication. The Analysis of date already published has revealed that, although the 3:4-benzo substituent has almost invariably been assigned a single δ value, there is apparently marked variation in the δ values required for different reactions. In the present investigation three reactions have been studied. The reactions were so chosen that (a) any past studies aided the interpretation of the present results and (b) the required δ values covered the range indicated by the analysis of data available in the literature. The reactions studied were as follows: (1) The reaction of benzoyl chloride with 1- and 2-naphthylanine in benzene was studied at 10°, 25°, 35°and 45°. (2) The solvolysis in ethanol of phenyl-1-naphthylcarbinyl chloride and of phenyl-2-naphthylcarbinyl chloride was examined at four temperatures over the range 5°-25°. For this reaction other data available were insufficient to obtain an accurate value of the reaction constant ρ, which is required for the calculation of a δ₃:₄-benzo value. Consequently the reaction kinetics of phenyl-m-bromophenyl-, phenyl-m-chlorophenyl-, phenyl-m-methylphenyl, phenyl-m-nitrophenyl- and diphenylcarbinyl chloride, were also examined in this reaction. These compounds were studied over the range 25°-75°. (3) The kinetics of the hydrolysis of ethyl 1-naphthyl-ethyl 2-naphthyl- and ethyl phenylacetate were investigated at four temperatures over the range of 20°-50°. It was clear from the present results, taken in conjunction with other data, that the δ values required for the 3:4-benzo substituent fall into significantly different classes. The difference may be linked ti the type and extent of conjugation possible between the substituent and the functional group in the reactant and transition states of these reactions. The reality of the differences in δ values, and the explanation for such differences, are discussed. For the reactions of 1-naphthyl derivatives studies in the present work, discussion of the 2:3-benzo substituent in terms of δ values is not meaningful without the accurate assessment of steric effects.