Comprehensive and accurate measurements of the speed of sound in liquid n-pentane and isopentane were carried with a double-path-length pulse-echo instrument. The measurements cover the temperature ranges between 200 and 420 K for n-pentane and between 220 and 420 K for isopentane with pressures up to 100 MPa. The expanded uncertainties (at the 0.95 confidence level) are 2.1 mK for temperature, 0.005% for pressure, between 0.01% at low pressure and 0.008% at high pressure for the speed of sound in n-pentane, and between 0.014% at low pressure and 0.01% at high pressure for the speed of sound in isopentane. At high temperatures and low pressure, the uncertainty in the speed of sound in n-pentane increases up to 0.026% at 420 K, while for isopentane it reaches 0.04%. In isopentane, strong damping of the sound signals was observed at temperatures below 260 K, which indicated dispersion of the sound waves in the frequency range of our measurements near 8 MHz. Measured speeds of sound in this temperature range were corrected for dispersion effects to the thermodynamic speed of sound at zero frequency by using information on the sound attenuation in isopentane from the literature. Due to the high damping at low temperatures, the uncertainty in the speed of sound in isopentane amounts to 0.022% at some state points on the isotherm 220 K.