Electronegativity equalization, which must accompany the formation of a chemical bond, occurs when electronic charge is transferred between the bound atoms but also by changes in the radial dependence of the atomic orbitals involved in the bonding. The degree of contraction or expansion of the atomic orbitals may be studied by analysing ab initio MO wavefunctions calculated with flexible basis sets. The effects on the hydrogen orbital are marked, the 1sHmean radius being progressively reduced by 9-23% across the series of first row hydrides (BH3 to HF) from its value in the hydrogen atom. The mean radius of the carbon 2p function in the wavefunctions of substituted methanes (CH3BH2 to CH3F) is correspondingly reduced by 2-19% from its free-atom value. Orbital contraction (or expansion) is dependent on bond distance, on the electronegativity difference of the bound atoms, and, because it varies from one MO to another, on the nature of the MO'S. The effects are greatest in MO'S which are strongly bonding.