The unexpected greater Lewis acidity of BCl3 than BF3 with respect to strong bases such as NH3 has been the subject of much discussion. A number of explanations have been proposed, among which the most popular and most widely quoted is that stronger back-donation from fluorine than from chlorine decreases the availability of the otherwise empty 2p orbital on boron from accepting an electron pair from a base. In contrast, toward weak bases such as CO, BF3 is a stronger Lewis acid than BCl3. We have reinvestigated the relative acid strengths of BF3 and BCl3 toward Lewis bases by calculating geometries and atomic charges for the following adducts: BF3. NH3, BF3. N(CH3)(3), BF3. OH2, BF3. O(CH3)(2), BCl3. NH3, BCl3. N(CH3)(3), BCl3. OH2, and BCl3. O(CH3)(2). Our results show that the halogen ligands remain close-packed throughout the formation of an adduct and that the bond lengths increase accordingly. It takes more energy to lengthen the short strong BF bonds than the longer weaker BCl bonds and it is for this reason that BCl3 is a stronger Lewis acid than BF3 toward a strong base such as NH3. In contrast, in the formation of a complex with a weak base such as CO, the BX3 is barely distorted from planarity and so the acidity of BF3 is greater than that of BCl3 because the charge on boron is greater in BF3 than BCl3.