In the summer of 2004, Jahn was appointed as Deputy Prime Minister for economic policy in the government of Stanislav Gross. After Gross' resignation, Jahn remained in Jiří Paroubek's government at the same position. In May 2005, he proposed a new economic agenda, by which he claimed the Czech Republic could catch up with western countries of the European Union in eight years. The agenda included the liberalisation of the public finance sector and tax policy, and the introduction of fees at universities. In his government function he also served as a chairman of the Council for human resources development and the government council for the support of research, development and innovations. He modernized the government support system, including accelerated depreciation for company investments into R&D.