The spectra of 43 aliphatic hydrocarbons have been measured between 3900 and 6000 cm(-1). it is shown that they can be represented as the sum of a limited set of group spectra, The required number of groups is established using principal factor analysis. A theorem is derived by which these group spectra can be determined directly from the eigenspectra (scores) and eigenvectors of the covariance matrix of the spectral data (loadings). Group spectra, integrated over 4 cm(-1) intervals, are presented for n-alkylmethylene and methyl, isopropyl, tert-butyl, 2,3-dimethylpropyl, and midchain methyl, ethyl and In,m-dimethyl. Using these the average error was 2.2% for the 35 compounds in the set for which the chain branchings were separated by more than one methylene unit. This corresponds to an average predictive error of 0.0004 molar absorbance units (mol dm(-3) cm), For CH2 chains with isopropyl units (six spectral the average error is 2.5%. These figures were comparable both with the deviations in the total integrated intensities as a function of the number of CH2 groups and also with the impurity levels (95-99% pure according to suppliers and to independent glc checks). We therefore consider that the additivity for remote groupings is good at the selected 4 cm(-1) resolution, Remote groups an defined as groups which are separated by two or more methylenes. Evidence is presented that the spectra of the functional groups are sensitive as to whether there is a -CH2- or a -CH3 group present in the beta position. For this reason the spectra of functional groups are extended to include corrections for beta CH3 groups, A rationalization of the effect of a beta methyl or, in general, of a group substituted at the beta position is given in terms of spectral perturbations of the CH2 adjacent to a methyl as compared to a CH2 group in the middle of a methylene chain. The principal features of the group spectra are interpreted. A major feature common to all terminal functional groups is explained in terms of changes in spectra of a midchain methylene and, of a methylene adjacent to a methyl or to another grouping. Evidence is presented for a reassignment of methyl bands in the 5900 cm(-1) region, based largely on a comparison of intensities in the spectra of n-alkylmethyl, isopropyl, and tert-butyl groups.