The influence of substituents and environment on the aromaticity of the naphthalene ring is shown for a series of peri- and para-substituted naphthalenes. Crystal structure geometries are compared with the single molecule structures in vacuum (optimized at the B3LYP/6-311++G** level) and with structures determined in media of different polarity. The harmonic oscillator model of aromaticity (HOMA) index of the naphthalene rings has been used to characterize the aromaticity of the investigated molecules. It has been shown that the ellipticity of the C2-C3 (C6-C7) bonds can be applied as a measure of participation of the quinoid resonance structure and through-resonance effect between the para-substituents.