The substituent effects on the trans/cis isomerization and the stability for the diazenes H-N(1)=N(2)-R (R = H, F, OH, OCF3, OCH3, CN, CHO, NH2, OH, CH3) have been studied at the B3LYP, MP4 and G3 level of theories. The results indicate that the charge variation on nitrogen atoms can be characterized by Hammett substituent constant and the electronegativity of substituent. The variation of the isomerization barriers can be attribute to the substitution induced fluctuations of the atomic charge in the N=N bond. The isodesmic reaction study suggests that the pi-donating ability of substituents dominates the stability of the diazene derivatives. (c) 2005 Elsevier B.V.. All rights reserved.