The effect of high temperature annealing on cation disorder in PbBi2Nb2O9 has been studied using a combination of powder neutron and anomalous dispersion X-ray diffraction methods. High resolution diffraction data shows that the orthorhombic cell volume is greater in rapidly cooled samples compared to slowly cooled samples. Rietveld analysis of the diffraction data gave the following cell parameters, space group A(21)am, a = 5.4909(1) and 5.4879(1), b = 5.4998(1) and 5.4989(1), c = 25.5313(2) and 25.5390(2) A for quenched and slowly cooled samples respectively. Anomalous dispersion X-ray diffraction methods have shown this is a result of disorder of the Pb2+ and Bi3+ cations. In the rapidly cooled sample a near statistical distribution of Pb2+ and Bi3+ in the 4a perovskite type sites was observed, the occupancies being 33 and 67% of Pb2+ and Bi3+ respectively. This occurs since both Ph2+ and Bi3+ have lone pair electrons and at high temperatures these cations have a similar preference for the 4a and 8b (Bi2O2) type sites. In the slowly cooled sample the 4a sites are occupied by 53 Pb2+ and 47% Bi3+ suggesting Bi has a preference for the 8b sites.