Investigations of temperature dependences of dynamic susceptibility for three ferrofluid samples with different size distributions are presented. These studies were performed at different sampling frequencies under the action of an external DC magnetic field. Analysis was done in terms of a Debye-like theory of complex susceptibility. It was also found that the temperature corresponding to maximum of the imaginary component of the complex magnetic susceptibility depends on the frequency of the probing field as a Vogel-Fulcher-like law.