Proton isotropic hyperfine splitting parameters determined by EPR and/or ENDOR are reported for the mono cation and anion radicals from the peri-alkyl-substituted naphthalene molecules: 1,2,3,6,7,8-hexahydropyrene, 1,2,3,4-tetrahydrocyclohepta[ de]naphthalene, 1,2,5,6,7,8-hexahydrocyclohepta[ fg]acenaphthylene, 2,3,6,7,8,9-hexahydro-l H-cycloheptal[ cd]phenalene, and 1,2,3,4,7,8,9,10-octahydrodicyclohepta[ de,ij]naphthalene, all in liquid solution. Assignments were made using empirical additivity rules, and the variations of the spectra with temperature resulting from oscillations of the alkyl rings. The additivity relationships were also used to evaluate the effect of spin density changes and naphthalene ring strain on the hyperfine parameters in the anion radicals.