Diffusion coefficients in air at 298.2 °K and 1 atm pressure are reported for ethane, ethene, ethyne, cyclopropane, propene, propadiene, propyne, butane, 1-butene, 2-methylpropene,cis-2-butene,trans-2-butene, 1,3-butadiene, 1-butyne, 2-methylbutane, 2,2-dimethylpropane, cyclopentane, and 1-pentene.A linear relationship, valid for alkanes and alkenes, has been found between the diffusion coefficient and the reduced molecular weight of the members of a homologous series and air.Diffusion coefficients calculated by ten prediction equations were generally lower than those observed.