The general aN index (GAI) was used to characterize the cis and trans isomers of hydrocarbons. The best one-variable equations were obtained with GAI for several physicochem. properties of seven pairs of olefin cis and trans isomers. Linear correlation coefficients were in the range 0.975 to 0.997. GAI was also compared with the other five topol. indexes, Randic connectivity index c, Wiener no. W, Hosoya index Z, the average distance sum connectivity J proposed by Balaban, and the aN index introduced by Yang, in correlating with the octane no. of heptanes and octanes.

Key words: ALKANE, ALKENE, ISOMER, OCTANE NUMBER, PHYSICAL PROPERTIES, HYDROCARBAN