Ground-state rotational spectra of the isotopomers H3P ...(BrCl)-Br-79-Cl-35, H3P ...(BrCl)-Br-81-Cl-35, H3P ...(BrCl)-Br-79-Cl-37, H3P ...(BrCl)-Br-81-Cl-37, D3P ...(BrCl)-Br-79-Cl-35, and D3P ...(BrCl)-Br-81-Cl-35, of the phosphine-bromine monochloride complex were observed by the pulsed-jet, Fourier-transform method, incorporating a mixing nozzle to preclude reaction among the component gases. Each isotopomer exhibited a symmetric-top-type spectrum which yielded accurate values of the spectroscopic constants B-0, D-J, D-JK, chi(aa) (Br), chi(aa) (Cl), M-aa (Br), and M-bb (Br) on analysis. Interpretations of the changes in the B-0 values with isotopomer showed that the intermolecular bond involves P and Br, with r(P ... Br)=2.869(1) Angstrom and that the BrCl bond increases in length by similar to 0.04 Angstrom on complex formation. Changes in the halogen nuclear quadrupole coupling constants when H3P ... BrCl is formed lead, with the aid of the Townes-Dailey model, to the conclusion that a fraction delta(i)=0.100(5) of an electron is transferred from P to Br on complex formation, while the polarization of BrCl by PH3 can be viewed as the transfer of 0.128(2)e from Br to Cl, leading to a net change of -0.028(5)e in the population of the 4p(z) orbital of Br. The complex is only of moderate strength, with an intermolecular stretching force constant k(sigma)=11.5 Nm(-1). Values of delta(i), similarly determined, for the series B ... BrCl, where B=CO, HCN, H2O, C2H2, C2H4, H2S, NH3, or PH3, are presented. It is shown that the variation of delta(i) with the ionization energy I-B of the Lewis base B can be described by an expression delta(i)=A exp(-bI(B)). This behavior is compared with that for the corresponding series B ... ICl.