Precipitation in a commercial duplex stainless steel, Zeron-100, has been investigated using conventional and high-resolution electron microscopy. Acicular fcc precipitates of Ni-rich n phase formed in the Cr-rich f matrix by suitable heat treatment. The orientation relationships between a large number of particles and the matrix were studied and found to be distributed in a range between the Kurdjumov-Sachs and the Pitsch relationships. All particles were observed to be bounded by two types of longitudinal facets, designated I and C, representing (...) 'incoherent' and 'coherent'. High-resolution microscopy showed that the I facet was based on the alpha / n incommensurate interface between Pitsch-related crystals. The C facet exhibited a coherent terrace structure with coherency strain parallel to accommodated by a disconnection array, causing the adjacent crystals to be slightly misoriented from the Kurdjumov-Sachs relationship. The orientational incompatibility between the I and C reference structures, 5.76° about , was found to be accommodated by networks of n -crystal dislocations superimposed on the facets. These acted as tilt walls of dislocations, but, although their combined angular tilt was always observed to equal about 5.7°, this was not partitioned between the I and C facets in a unique manner. Such differential partitioning of the incompatibility offers a natural explanation for the observed variation in particle orientation. Some issues, such as the growth mechanism, were not resolved and need further work. (shrink)