Quantitative measurements of the absorption spectra of the six isomeric pentenes in vapor phase have been made in the region 45 000—65 000 cm—1. The maximum of the intense band (N→V) is the same for 1-pentene, cis-2-pentene, and 2-methyl-2-butene, with one, two, and three alkyl substituents, respectively. The results show that in a series of hydrocarbons, where the molecular weight is unchanged and the number of alkyl groups is the only variable, the position of the N→V maximum is not a function of the number of alkyl substituents. From the standpoint of methyl substituents, there is no shift between cis-2-pentene with one methyl group and 2-methyl-2-butene with three. In the absence of such a shift, spectral evidence in support of hyperconjugation in the substituted ethylenes is lacking. The N→V maximum for trans-2-pentene is less intense and at somewhat longer wavelength than for the cis-isomer. The greatest shift in this band is with 2-methyl-1-butene, where the two alkyl groups are on the same carbon atom. These results indicate the importance of molecular shape in determining the energy of the N→V transition in the monoolefins. The positions of the shallow bands (N→R) on the long wave side of the N→V band are in substantial agreement with earlier measurements and confirm the conclusion that the energy of this transition is dependent on the number of alkyl substituents on the carbon atoms of the double bond. The possibility of other Rydberg bands on the short wave side is discussed briefly. Oscillator strengths for the N→V band are given for the six pentenes; these vary from 0.26 in 2-methyl-1-butene to 0.45 in cis-2-pentene but their value for testing theoretical predictions is doubtful because of the overlapping of different electronic transitions.