An objective of this paper is to determine the jump in plasma parameters across the November 12, 1978, interplanetary shock, sufficiently accurately to test in a subsequent paper a major prediction of shock acceleration theory: the dependence of the energetic ion spectral index upon the density compression ratio. We use ISEE 1 and 3 measurements of the magnetic field and electron and proton densities, temperatures, and bulk velocities, as well as ISEE 3 alpha particle measurements, and confirm the ISEE 1 electron densities using plasma wave measurements. We solve for the shock normal using four independent methods and show that the upstream and downstream flow parameters are consistent to better than 10% with &ggr;=5/3 Rankine-Hugoniot jump conditions. We conclude that the November 12, 1978, shock was a high-speed (612 km s-1), supercritical, quasi-parallel (&thgr;Bn=41¿) shock of moderate strength (fast Mach number of 2.8) propagating into an upstream plasma whose total &bgr; was 1.14 and whose electron-to-proton temperature ratio was 2.8. This shock had three dissipative scale lengths, one of a few Larmor radii associated with its magnetic field jump, one of about 10 RE associated with electron equilibration, and one of about 30 RE associated with an energetic proton foreshock.