M2C carbide precipitation was investigated in a martensitic Co-Ni steel, AF1410. Results of TEM (from both thin foils and extraction replicas) and X-ray diffraction were combined with results of collaborative SANS and APFIM studies to determine phase fractions, compositions, and lattice parameters throughout precipitation, including estimation of carbide initial critical nucleus properties. The composition dependence of the M2C lattice parameters was modelled to predict the composition-dependent transformation eigenstrains for coherent precipitation; this was input into collaborative numerical calculations of both the coherent carbide elastic self energy and the dislocation interaction energy during heterogeneous precipitation. The observed overall precipitation behavior is consistent with theoretically-predicted behavior at high supersaturations where nucleation and coarsening compete such that the average particle size remains close to the critical size as supersaturation drops. The precipitation in AF1410 at 510°C exhibits a 'renucleation' phenomenon in which a second stage of nucleation occurs beyond the precipitation half-completion time (1-2hrs).