The CNDO approximate molecular wavefunctions for BCl3 have been applied to the calculation of the derivatives of the dipole moment with respect to the symmetry coordinates. New experimental derivatives calculated from previously published intensity data are presented. The set of experimental derivatives with respect to internal coordinates definitely preferred by these calculations is that with all signs negative. Inclusion of the d Orbitals of chlorine in the atomic orbital basis set markedly influences the magnitudes of the derivatives as well as improves the agreement with the experimental in-plane and out-of-plane bending derivatives. The pd polarization terms appear to play an important role. Our results for BCl3 are compared with those previously obtained for BF3. 43624366