We have examined size-dependent electronic relaxation dynamics in isolated semiconducting mercury cluster anions using time-resolved photoelectron imaging. Relaxation following excitation from within the conduction (p-) band occurs on an ∼3–40 ps timescale and is attributed to non-adiabatic relaxation through the p-band. Exciting an electron from the valence (s-) band into the conduction band creates an electron–hole pair that relaxes prior to Auger emission. The dynamics associated with this feature occur on an ∼500 fs timescale and are attributed to either a hole-induced contraction of the cluster or electron–electron scattering.