Measurements of the complex susceptibility, $\chi(\omega)=\chi'(\omega)-{\rm i}\chi''(\omega)$, as a function of the frequency (100 Hz–18 GHz) and polarizing field (0–90 kA m−1) at room temperature together with static magnetic measurements over the temperature range 4–300 K, are reported for a colloidal suspension of cobalt nanoparticles. The transition of the cobalt particles to the superparamagnetic state are supported by the temperature dependencies of field cooling (FC) and zero field cooling (ZFC) magnetization measurements. From these measurements, which show a typical blocking behaviour of an assembly of superparamagnetic particles with a wide distribution of blocking temperatures, the exponential pre-factor $\tau_0$ of Brown's equations for Néel relaxation, is found to be equal to 9.2 × 10−10 s. Measurement of the complex susceptibility $\chi(\omega)$ over this broad frequency range, with an upper frequency value corresponding to three times that previously reported in our measurements on cobalt, has enabled the presence of both resonance and Néel relaxation mechanisms to be identified. From the Néel component, a further value for $\tau_0$ was evaluated and shown to be in close agreement with that obtained from the ZFC data. Data on the after-effect function, realised by Fourier transformation of the $\chi''(\omega)$ component, is also presented.