Experimental vapor pressure data reported in the literature for twelve nonane isomers have been rigorously analyzed and used to determine the constants A, B, C, D, and exponent of the following vapor pressure equation in the form of reduced vapor pressure and reduced temperature, proposed by Zia and Thodos: logP_r=A+B/T_r+C/T_r²+DT_r³ According to varying exponent n all four constants have been obtained for nonane isomers by the error analysis. This has provided us with the best n and four constants for each material. In order to obtain the calculated vapor pressure by the above equation for each constants, only normal boiling point and the critical pressure and critical temperature are necessary to get an overall average deviation of 0.23% for 445 experimental vapor pressure points consisting of twelve nonane isomers available in the literature.