Abstract.B3LYP calculations in conjunction with natural bond orbital population analysis have been performed for a previtamin D model and corresponding transition structures for the [1,7]-hydrogen migration. In addition the 19,19-difluoro, 19-methoxy and 19-fluoro substituted analogs were investigated. The calculated activation barriers decrease in the following order: CHF2CH3CH2OCH3 (24.8, 23.5 and 20.1 kcal/mol). This is in qualitative agreement with experiments. It has been suggested that a decrease of the barrier by a 19-methoxy substituent and its increase by a 19,19-difluoro substituent are phenomena of different origin. In the case of 19-methoxy substitution, the effect is due to the charge redistribution in the triene system and the decrease of the C(19)H bond energy. The effect of two fluorine substituents at C-19 on the activation barrier is suggested to originate from the combination and balance of several factors: electrostatic repulsion between the negative fluorine atom and the -electron cloud over the conjugated system, an increase of the HOMOLUMO gap, and geminal difluoro substitution affecting CF and CC bond energies.