The defect structure and transport properties of complex perovskites with the general formula Sr-3(Sr1+xNb2-x)O9-3x/2 have been studied through the use of thermogravimetry (TG), ac-conductivity and transport number measurements, comprising samples in the compositional range between Sr-3(SrNb2)O-9 (x = 0) and Sr-3(Sr1.5Nb1.5)O-8.25 (x = 1/2) (Sr/Nb ratio varying from 2 to 3). TG was used to measure the uptake of water, and the highest reversible water uptake was found for the end member Sr-3(Sr1.5Nb1.5)O-8.25 (x = 1/2). The total conductivity (as a function of pH(2)O, pO(2) and T) for a sample of composition Sr-3(Sr1.4Nb1.6)O-8.4 was fitted to a model based on Sr-acceptor defects compensated by doubly charged oxygen vacancies and protons. The partial conductivities found by transport number measurements support the modelled results. Oxygen ion conductivity dominates at high temperatures and protonic conductivity dominates below similar to 550 degrees C at ambient humidities. The enthalpy and entropy for the reaction of water with oxygen vacancies to form protons were found to be -160+/-5 kJ mol(-1) and -166+/-5 J K-1 mol(-1), respectively, for this sample. Oxygen ion and proton conductivities were also found to increase with Sr content. Moreover, the increase far exceeds the expected behaviour, and possible causes for this are discussed.