The susceptibility of an array of fine nickel particles was measured as a function of temperature from 1.5 to 300 °K at a frequency of 5 kHz. The particles were formed by cosputtering nickel with SiO2 or Al2O3 and varied in diameter between 10 and 100 Å. With increasing temperature, the susceptibility increased from its initial value to a maximum at a temperature TB followed by a hyberbolic decrease. A theory based on the relaxation time for single-domain particles τ=τ0 eK Vk T (K is the anisotropy energy, V is the particle volume) quantitatively explains the data. At T=0 all the particles are blocked by the anisotropy barriers. As the temperature is increased the susceptibility increases because particles for which ωτ<1 (ω is the angular frequency) are no longer blocked. Above TB all the particles are unblocked and the susceptibility is characteristic of superparamagnetism. Analysis of the data yields information about the particle volume distribution function and a value for the effective anisotropy energy.