Lead monoxide exists at room temperature in two forms, namely α-PbO, litharge (red, tetragonal) and β3-PbO massicot (yellow, orthorhombic). The weak bonds along the c axis and the ferroelastic nature of the incommensurate phase prevent the growth of single crystals suitable for high quality diffraction techniques (very small ferroelastic domains and frequent stacking faults in these layered structures). A structural determination of the incommensurate structure was proposed in the superspace group C2mb(0β;0) using the integrated intensities extracted from powder diffraction patterns. More recently an electron microscopy study suggested the existence of systematic extinctions affecting the satellite reflections and it supports the more symmetric space group Cmma(0β0)s. In this paper, high resolution powder diffraction patterns (synchrotron) were refined in the incommensurate phase using the Rietveld method. Various superspace groups and forms for the modulated displacements were tested. The consequences of this structural model on the electronic structure are analysed using also ab-initio calculations.