Differential travel times, commonly used to eliminate unwanted near-source and near-receiver heterogeneity from travel time studies, can be affected by slabs and hypocentral errors when phase pair slownesses differ greatly. In particular, D' and core structure studies can be biased if differential PKP times are used. Here we present differential PKP travel time measurements having significant azimuthal travel time anomalies consistent both in size pattern with a near-source slab effect and explore other sources of error in differential times. ¿ American Geophysical Union 1994