The author set forth an objective to identify, classify and evaluate the sources of studying the Social Democrats’ activities in the State Duma of the Russian Empire of the I and II convocations. The memoir and archaeographic sources devoted to the activities of the Social Democrats in the State Duma, the archival funds, the verbatim records of the Duma and the publications in the periodical press on the activities of the Social Democrats in the Duma are included. The comparative analysis and classification of the groups of sources for their informative value on the topic is performed. The sources to which the researchers pay undeservedly little attention are identified, as well as the archaeological publications on the topic are evaluated for their representativeness. The theoretical / practical significance is in the novelty of the formulation and solution of the question of the source base of the study of the Social Democrats’ Duma activities and the possibility of using the factual material, the main provisions and conclusions of the article in writing special and generalizing works of both source studies and historical and political science. The article is addressed to historians and political scientists dealing with the history and source studies of parliamentarism in Russia.