In order to elucidate the nature of heterogeneous nucleation, a differential scanning calorimetry (DSC) thermal analysis of Fe–10mass%Ni alloys containing oxide catalysts was conducted. For all oxide catalysts, the maximum frequency in the particle-size distributions diminishes and the modal value increases with a decrease in the amount of deoxidizers or with an increase in the elapsed time after deoxidizer addition. That is, the number of smaller particles decreases, while the number of larger particles increases. In proportion to this change of the particle-size distribution, the critical undercooling (ΔT) increases. For the same amount of deoxidizers and the same elapsed time, the value of ΔT for the primary crystal of the γ phase is smaller in the order of MgO, ZrO₂, Al₂O₃, and CaO–Al₂O₃. Oxide catalysts grow by Ostwald ripening, therefore the growth can be controlled by the diffusion of oxygen. Thus, the decrease of the soluble oxygen content by strong deoxidizers results in the inhibition of particle growth.