The speed of sound has been measured in two methane-rich gas mixtures having nominal compositions of(0.85CH(4) + 0.15C(2)H(6)) and(0.80CH(4) + 0.10N(2) + 0.05C(2) H-6 + 0.03C(3)H(8) + 0.02CO(2)). The measurements were made with a spherical acoustic resonator on four isotherms at T/K = 250, 275, 300, and 350 and at pressures between p = 0.1 MPa and p = 20 MPa. The final results for the two mixtures have an estimated overall uncertainty of +/-0.03 per cent. The speeds of sound in the limit of zero pressure have been estimated by extrapolation and the results generally agree with calculated values to within +/- 0.01 per cent. The experimental values for the speed of sound are compared with the predictions of the 1992 AGA8 equation of state. The results do not differ by more than 0.2 I,er cent at temperatures above T = 275 K but deviations of up to 1 per cent are found on the isotherms at T = 250 K.