Unique worldwide, Lake Kivu stores enormous amounts of CH4 and CO2. A recent study reported that CH4 concentrations in the lake have increased by up to 15% in the last 30 years and that accumulation at this rate could lead to catastrophic outgassing by ∼2100. This study investigates the present-day CH4 formation and oxidation in Lake Kivu. Analyses of ¹⁴C and ¹³C in CH4 and potential carbon sources revealed that below 260 m, an unusually high ∼65% of the CH4 originates either from reduction of geogenic CO 2 with mostly geogenic H2 or from direct inflows of geogenic CH4. Aerobic CH4 oxidation, performed by close relatives of type X CH4-oxidizing bacteria, is the main process preventing CH4 from escaping to the atmosphere. Anaerobic CH 4 oxidation, carried out by CH4-oxidizing archaea in the SO4^2--reducing zone, was also detected but is limited by the availability of sulfate. Changes in ¹⁴CCH4 and ¹³CCH4 since the 1970s suggest that the amount of CH 4 produced from degrading organic material has increased due to higher accumulation of organic matter. This, as well as the sudden onset of carbonates in the 1960s, has previously been explained by three environmental changes: (1) introduction of nonnative fish, (2) amplified subaquatic inflows following hydrological changes, and (3) increased external inputs due to the fast growing population. The resulting enhancement of primary production and organic matter sedimentation likely caused CH4 to increase. However, given the large proportion of old CH4 carbon, we cannot exclude an increased inflow of geogenic H2 or CH4. Copyright 2011 by the American Geophysical Union.