We analyze carbon intensity performance at firm level and the effectiveness of the Swedish CO2 tax. Carbon intensity performance is derived from a production technology and measured as changes in the CO2 emission-output production ratio. As one of the first countries to introduce a CO2 tax in 1991, Sweden serves as an appropriate "test bench" for analyzing the effectiveness of climate policy in general. Firm level data from Swedish manufacturing spanning over the period 1990-2004 is used for the analysis. Results show that EP has improved in all the sectors and there is an evidence of decoupling of output production growth and CO2 emissions. Firms' carbon intensity performance responds both to changes in the CO2 tax and fossil fuel price, but is more sensitive to the tax. (c) 2014 Elsevier Ltd. All rights reserved.