Ab initio calculations on the protonation of substituted phosphines are used to study the relationship between proton affinities and lone pair density properties. It is assumed that these properties are conveniently represented at the point of maximal concentration of non-bonded charge in the phosphorus valence shell of charge concentration, that is, at the (3,-3) critical point in del(2) rho. These lone pair properties appear to be an excellent indicator for the proton affinity of alkyl phosphines. However, the same properties fail to recover even the qualitative trend in proton affinities of halo-phosphines. In addition, it is shown that the (3,-3) critical point properties can predict the incorrect site of preferential protonation in halo-substituted phosphines containing distinct trivalent phosphorus atoms. These results are rationalized in terms of Bader’s Atoms-In-Molecules theory. It appears that integrated properties (atomic charges and energies) may give more insight into the mechanism of substituent effects on basicity than del(2) rho or the LP properties.