The interaction of an active comet with the solar wind is effected by the pickup of heavy cometary ions over extended regions of space. It was predicted before any spacecraft encounters with comets that a bow shock would form at a critical point in the mass-loaded flow. Here we present an analysis of the results from Giotto's encounters of comets Halley and Grigg-Skjellerup on the plasma structure of the bow shock regions at the two comets. This is achieved in a joint analysis of data on solar wind and cometary ions, magnetic field, and electrons. We find that these mass-loading shocks are among the most complex in the solar system with heavy cometary ions dominating the component pressures. Results of Mach number calculations at Halley support earlier interpretations that bow shocks were, indeed, seen with possible solar wind disturbance inbound. At Grigg-Skjellerup, Mach number results support the interpretation of a wave inbound and a shock outbound; both features also occur near to the expected location for a bow shock. A weak correlation between magnetic field strength and electron density is noted at the Grigg-Skjellerup outbound shock in agreement with quasi-perpendicular hybrid simulations.¿ 1997 American Geophysical Union