A complex technological investigation of 14th-18th centuries Venetian paintings from the collection of the National Museum in Warsaw has been performed in connection with the Serenissima – Light of Venice exhibition. After analysis of the X-ray radiographs of the paintings, several of them had a complex structure which justified the use of neutron-induced autoradiography to obtain indisputable information about over-painting (Tintoretto) or either confirmation or contradiction of their authenticity (Marieschi, Bellotto). All the paintings under analysis were irradiated in a specially designed station at the research reactor MARIA in Swierk near Warsaw. Beta-rays emerging from the painting surface irradiated by thermal neutrons and recorded on a X-ray film displayed the distribution of elements in pigments used for creating individual layers of the painting. It allows for tracing the particular phases of the painting structure invisible to the naked eye. The report presents results obtained for the following paintings: J. Tintoretto – Portrait of a Venetian Admiral, M. Marieschi – The palace of Doges in Venice, B. Bellotto called Canaletto – Fantasy architecture with self-portrait of the artist.