Ab initio quantum chemical calculations are performed, within the Double Harmonic Approximation, on the integrated intensities of infrared absorption bands associated with the fundamental transitions in the pyramidal AX 3-type molecules: NH 3, PH 3, NF 3 and PF 3. The (∂μ/∂ Q i ) 0 values needed were calculated using a finite difference method employing the (3.1) basis published by Maroulis et al. [27] which can be considered as a compromise between quality and cost. When comparing these theoretical values with experimental intensity values, a substantial improvement is obtained in comparison to the results of previous calculations employing a Double Zeta basis; the average ratio between theoretical and experimental derivatives decreasing from 2.08 to 1.52. The (3.1) results are close to those obtained using a much larger (DTZP) basis, previously used in the case of the simplest molecule NH 3 [22]. The experimental intensity sequences (a) for various modes within a given molecule, and (b) for a given mode throughout the series NH 3, PH 3, NF 3 and PF 3 are correctly reproduced at the (3.1) level. An analysis of the combined influence of purely vibrational and electronic factors on the calculated intensities is performed, showing, for example, that the A 1 sequence (NH 3 < NF 3) is essentially vibrational in origin, whereas the A 2 sequence (PF 3 > NF 3) is due to electronic effects. An analysis of the dipole moment derivatives (∂μ/∂ Qi) 0 in terms of contributions from Localized Molecular Orbitals [18] is performed with special attention being paid to the symmetric bending mode Q 2 in view of its high vibrational purity and the possible importance of an (incomplete) orbital following effect with molecular distortions along this normal coordinate. The results of a study of the behaviour of the lone pair on the A atom upon deformation of the molecule along Q 2 can be considered as a non-empirical support for the McKean and Schatz model [23a], based upon classical geometry-hybridization relationships, and relating the moment of the lone pair to the XAX angle, α, for a given molecule. It is shown how the complete series of molecules considered can be fitted into this scheme. Inspection of the behaviour of the AX bond LMO's following molecular deformation along Q 2 gives a clear insight into the phenomenon of incomplete orbital following and its influence on the ν 2 intensity.