The kinetics and the effect of indigenous and externally added water on methane formation during Fischer-Tropsch synthesis (FTS) was studied over Co based catalysts using a 1 L continuously stirred tank reactor (CSTR). The water cofeeding study (10% water) was conducted over a 0.27%Ru-25%Co/Al2O3 catalyst at a low CO conversion level of 19% at 220 degrees C in order to lessen the effect of catalyst aging during the addition of water, while the kinetic experiment was conducted over 25%Co/gamma-Al2O3 at the conditions of 205-230 degrees C, 1.4-2.5 MPa, H-2/CO = 1.0-2.5, and 3-16 (NL/g(cat))/h (X-CO < 60%). Indigenous and externally added water decreases methane formation by a kinetic effect. The addition of 10% water led to a decrease in the CH4 rate by 12% (3.5 -> 3.0 (mmol/g(cat))/h), while little catalyst deactivation was observed during water addition. Increases in indigenous water partial pressure also lowered the CH4 rate and its selectivity. Kinetic analysis was performed using a group of 220 degrees C data collected between 365 and 918 h when the deactivation rate was very low. An empirical CH4 kinetic model, with a water effect term (P-H2O/P-H2), (r(CH4) = kP(CO) P-a(H2)b/(1 + mP(H2O)/P-H2)) was used to fit kinetic data. The CH4 kinetic results suggest a negative water effect on CH4 formation during FTS on the unpromoted cobalt catalyst, consistent with the water effect results. The final methane kinetics (r(CH4)) equation obtained at 220 degrees C over 25%Co/gamma-Al2O3 is as follows: r(CH4)/[(mol/g(cat))/h] = 0.001053{(PCO-0.86PH21.32)/[1 + 0.46(P-H2O/P-H2)]}. Meanwhile, a methane selectivity model at 220 degrees C for the 25%CO/Al2O3 catalyst was also developed: S-CH4 = 0.0792P(CO)(-0.55)P(H2)(0.44)[1 - 0.24P(H2O)/P-H2)/(1 + 0.46P(H2O)/P-H2)]. The CH4 selectivity model provided a good prediction of CH4 selectivities under the experimental conditions used. Furthermore, our empirical CH4 kinetic results on the cobalt catalyst are consistent with literature kinetic models that were derived from carbide mechanisms; high CH4 selectivity from the cobalt catalyst is found to be mainly due to a high CH4 reaction rate constant.