The results of magnetization measurements on ∼3 nm amorphous (Co0.25Ni0.75)65B35 nanoparticles, in a diluted dispersion (4% in a polymer) and in a powder form, are reported. The results of M(T) and M(H) measurements give evidence of an important surface contribution to magnetization, which is responsible for a non-saturated component in the magnetization curves and for some anomalies in the coercive field and temperature dependence of the remanence for the sample of dispersed particles, where interparticle interactions can be neglected. The results have been interpreted with a simple model where each single-domain nanoparticle is considered as a core–shell system, with uniaxial anisotropy on the core and surface anisotropy on the shell.