The article «Applying Google Trends to study the German legal deposit copy system» discusses the use of web tools to investigate current library science problems. Using web-based statistical method the author searches the following issues: the dynamics of interest in the subject of a legal deposit copy from the date of adoption of the Law on the German National Library (2006) to nowadays; relations of the public interest peak changes in this topic to certain phenomena in the social and cultural life of Germany; the federal dimension of these issues when comparing interest to the topic in different regions of Germany; the public opinion on the popularity of legal copy among traditional and electronic sources. The article is divided into four parts. The first one sets the work objective and main tasks, gives a general description of the chosen research method. The second part deals with the process of creating a semantic dictionary; analyzes traditional and electronic sources of synonymic dictionaries. It describes the strengths of such an online language matching service as semager.de. The third part dissects a group of keywords related to the topic of a legal deposit copy along with the most interesting and problematic, from the point of the author’s view, additional keywords such as the German National Library, network publications, and dissertations. Using web statistical tools the paper shows that the most intense issues regarding the legal deposit, the problems of the German National Library and online publications were raised in the lands of West Germany. Developing the legal deposit copy system will go, first of all, through online publications, greater cooperation with academic and scholar libraries; open access of scientific data and publications related to dissertations and theses of West German lands’ universities. The fourth part presents main conclusions and substantiates the method significance for library and sociological research.