Phosphorescence and TREPR spectra have been measured for 2-methyl-1,4-naphthoquinone (MNQ), 2,3-dimethyl-1,4-naphthoquinone (DMNQ), and 2-methoxynaphthoquinone (MeONQ) in several matrices at low temperatures. The \D\ value of ZFS parameter decreases with increasing the electron-donating character of the substituent group. For MNQ, the T-1 state is clearly assigned to be (3)n pi* in character. The broad phosphorescence spectra and minor effects of the matrix polarity on the ZFS parameters lead to the conclusion that the character of the T-1 states are mainly 3 pi pi* in DMNQ and MeONQ. The T-1 states of the present naphthoquinone derivatives have more or less mixed character of the 3n pi* and 3 pi pi* states.