High resolution measurements on the J = 1 <- 0 transitions of PbO have been performed using a Fourier transform microwave spectrometer. The available transition frequency for the (PbO)-Pb-208-O-18 isotopomer has allowed the rotational molecular field shift parameter for lead, V-01(Pb), in PbO to be determined for the first time. The parameter gives unique insight concerning how the electron density at the Pb nucleus alters as a function of the Pb-O internuclear separation. (c) 2007 Elsevier B.V. All rights reserved.