Time-series of estimated monthly carbon dioxide emissions from consumption of coal, petroleum and natural gas in the United States from 1981 to 2002 have been derived from energy consumption data. The data series for coal and natural gas each reveal a consistent seasonal pattern, with a winter peak for gas and two peaks (summer and winter) for coal. The annual cycle of total emissions has an amplitude of about 20 Tg-C, and is dominated by CO2 released from consumption of natural gas. Summation of the monthly estimates to obtain annual values reveals good agreement with other estimates of CO2 emissions. The varying proportions of CO2 emitted from each fuel type over the course of a year lead to an annual cycle in the carbon isotope ratio (δ13C), with a range of about 2 ‰. These monthly carbon emissions estimates should be helpful in understanding the carbon cycle by providing (1) monthly/seasonal input for carbon cycle models, (2) estimates of the annual cycle of the 13C isotope ratio in fossil-fuel CO2 emissions and (3) data at fine enough time intervals to investigate effects of seasonal climate variations and changes in seasonally dependent use patterns of certain appliances (e.g. air conditioners) on fossil-fuel carbon emissions.