The speed of sound in n-octane and isooctane (2,2,4-trimethylpentane) was measured by the Brillouin light scattering (BLS) method. The examined regions for n-octane are T = (297-580) K along five isobaric lines at p = 0.1, 4.0, 7.0, 10.0, and 12.0 MPa for liquid and T = (573-673) K along ten isobaric lines with p = (3.0-10.0) MPa for supercritical fluid. The examined regions for isooctane are T = (294-525) K along five isobaric lines at p = 0.1, 3.0, 6.0, 10.0, and 12.0 MPa for liquid and T = (543-630) K along six isobaric lines with p = (3.0-10.0) MPa for supercritical fluid. The relative expanded uncertainty of the speed of sound is estimated less than 1.3%. Polynomial representations for the speed of sound in liquid n-octane and isooctane were fitted to the experimental results, respectively. The AADs are 0.27% for n-octane and 0.19% for isooctane. The influence of temperature and pressure on the speed of sound was also analyzed. Moreover, the data were also used to assess the predicted ability of three equations of state for n-octane.