This paper is an account of an extensive series of investigations on ionospheric cross-modulation (wave interaction) in special tests arranged in collaboration with the British Broadcasting Corporation. Special attention is paid to seasonal and nocturnal changes in the cross-modulation and to the theoretical interpretation of the experimental results. It is shown how a variety of evidence is consistent with the assumption that the cross-modulation is highly localized in the E-region at one or two regions near the lower of the surfaces of reflexion of the wanted and disturbing waves. An estimate is made of the gradients of electronic concentration and collisional frequency at a height of about 86 km. Certain theoretical formulae used in the paper are derived in the appendix.