The surface chemistry of nickel ultrafine powders (UFPs) was examined by temperature programmed desorption (TPD) measurements. Nickel UFPs exposed to air contained NiO and Ni(OH) 2 . The oxide phase increased during heating in vacuum mainly due to the adsorbed gases and to the decomposition of the Ni(OH) 2 . Sintering characteristics of three types of nickel UFP in vacuum and a stream of hydrogen were examined by measuring their dimensional changes, specific surface areas, crystallite sizes, and structural changes. In the case of vacuum sintering of clean nickel UFPs, neck growth by surface diffusion occurs even at room temperature and densification by grain boundary and/or lattice diffusion starts below 400 K