The article deals with the diaries as an important narrative source for historians in their study of the past. The diaries are a very peculiar type of source that is different from memoirs. Usually, the author writes his memoirs about the events that he lived through many years ago or he witnessed in the past. It is naturally that with the flow of time he may forget about something, miss something or recall something incorrectly; moreover, as it is known, in memoirs events of the past are reflected with a hindsight and that is why they do not always accurately reflect the feelings of the author at the time when these events took place. People’s attitudes towards the events can also change as the years go by. Moreover, human memory is very much selective. Unlike memoirs, the diaries allow us to understand the attitude of a particular human being to the surrounding world and contemporary events when he or she wrote about them. This has enormous value for the researcher. The diaries, as a type of historical document, are important precisely because they provide us not only with object of a historical process, in other words, what is being described, but also with insights about their authors who are present not as abstract depersonalized subjects of this process, but as living human beings with their merits and shortcomings, with their convictions and moods, their dreams and concerns. Moreover, the diaries provide unique material for the study of social psychology of people in concrete historical period. The author of diaries, which became the object of our attention, was Vladimir Ivanovich Vernadskyi (1863-1945), an outstanding Russian 20th century naturalist, geologist, biogeochemist, thinker and winner of the Stalin Prize, academician, creator of the doctrine of the biosphere, its evolution and transformation into the noosphere, who was one of the founders of the Ukrainian Academy of Sciences and its first President, as well as the founder of the National Library of Ukraine. Their main value of his diaries, in our opinion, is that they help to feel the atmosphere of the dramatic events of October Revolution and Civil War in Russia and Ukraine, allows us to restore the spirit of the revolutionary stormy years in all its tragedy and contradictions. As for the content of the great scientist’s diaries, a significant part of them consists of the facts of his personal life., but also a lot of space is devoted to his impressions and reflections on political events, in particular about the German and the Austro-Hungarian occupation of Ukraine in 1918. In his diaries, a negative attitude towards the Bolsheviks and socialist ideology, non-acceptance of manifestations of Ukrainian nationalism, and condemnation of the talentless policy of the Central Rada can be clearly traced.