Summary A statistical treatment of observations of SKS is combined with other information on SKS and on the AB branch of PKP, leading to a revised velocity distribution for the outer core. The seismic velocity at the top of the core appears to be 8.26 km s-', somewhat greater than Jeffreys's value of 8.10 kms-'. The distribution of seismic velocities in the Earth's outer core has until recently been only weakly constrained by observation. The core velocities are obtained by applying the Wiechert-Herglotz method to the travel-time curve appropriate to an earth model stripped of its crust and mantle by subtracting from the travel times and distances of SKS those for ScS at equal values of the ray parameter p = dt/dA, and by similarly subtracting PcP from PKP. To obtain values for the outer core we therefore need observations of the AB branch of PKP, and of SKS together with a knowledge of the distribution of S-wave velocities in the mantle. All of these topics have been investigated recently. Bolt (1968) and Gogna (1968) have made studies of the AB branch of PKP. Randall (1970) has studied travel times of S, and by combining his observations with studies of S diffracted around the core, has deduced a velocity distribution for S waves compatible with that of Herrin et al. (1968) for P. Hales & Roberts (1970) have studied the travel times of SKS and further work on SKS is reported here. This information can be used to obtain revised estimates of the velocity distribution in the outer core. 2. Observations of SKS The statistical methods used by Randall (1970) to obtain a revised table of travel times for S, that is consistent with that of Herrin et al. (1968) for P, have now been applied to SKS. Observations of SKS in the distance range 97.5 to 118.5 for the earthquakes listed by Herrin et al. (1968) were obtained from the International Seismological Centre Bulletins, and grouped into 1 distance ranges. The mean travel time for each distance range was calculated by an iterative process incorporating truncation of the data at twice the estimated standard error to remove gross errors. In the intervals centred on 99, 107 and 110 the reported observations were so scattered that the process did not converge, and no mean values are available for those ranges. Jeffreys' method of summary values was used for smoothing the mean travel times. With the intervals divided into two groups, and consequently with 14 degrees of freedom, x2 was 10.0 and within the limits for satisfactory smoothing. Numerical results are shown in Table 1. The precision of the smoothed travel times is indicated by the estimated standard error of the summary times and is about half a second.