Metal injection molding (MIM) is an attractive alternative for net-shape manufacturing of components with high demands on shape complexity. Spherical powders with small particle sizes and a wide distribution are generally preferred for MIM parts. In this study, the sintering behavior of an annealed spherical carbonyl iron powder, a standard carbonyl iron, and a sieved (-45 ?m) water-atomized iron powder was investigated. Injection molded compacts made from the respective powders were sintered in a dilatometer in a hydrogen atmosphere to study the shrinkage behavior. The shrinkage of the powder during heating stage was related to the surface area, morphology and surface chemistry of the powders. Surface composition of the powders was studied by X-ray photoelectron spectroscopy (XPS) and high-resolution scanning electron microscopy (HR SEM) combined with energy dispersive X-ray analysis (EDX). Results show strong correlation between the sintering activity to the surface chemical composition of the powder and surface area.