The unpublished letters from Nicolas I, Alexander II and their relatives fetched USD 835,000 -- far above the estimate of 60,000-80,000 francs.
According to auction house Hotel Des Ventes (HDV), most were purchased by the same buyer at yesterday night's sale.
The letters, written in Russian, French and English, were sold by an American who had received them from the widow of a US captain deployed in Europe during World War II.
Most were addressed to Grand Duchess Olga Nicolaievna, the daughter of Nicolas I and sister of Alexander II, who moved to Stuttgart after marrying a German prince -- later to become King Charles I of Wurttemberg.
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Through the missives, "intimate secrets and plots of the Russian Court are unveiled as well as the beliefs and political pains of the two emperors," HDV said.
The auction house meanwhile said it had, upon request from the Geneva judiciary, withdrawn from sale an unpublished set of photographs of Alexander III's family.
HDV says the snaps were rescued by Otto Hofmann, a German Bauhaus artist who was serving with the Nazi forces when they destroyed the Gatchina palace in 1944.
"Acting against martial law and putting his own life at risk, the soldier hid and brought back around 30 images which he considered historical evidence and culturally important for Imperial Russia," the auction house explained.