We report on a combined experimental and theoretical study of the ionization energies and structures of small lithium doped silicon clusters, Si_nLi_m with n = 5–11 and m = 3–6. Photoionization efficiency curves are measured in the 4.68–6.24 eV range and subsequently compared with calculated values of both vertical and adiabatic ionization energies for the lowest energy isomers obtained using density functional theory at the B3LYP/6-311+G(d) level. The evolution of the cluster geometries and ionization energies is studied as a function of the number of silicon and lithium atoms along the Si_nLi₃ (n = 5–11) and Si₈Li_m (m = 1–6) series, respectively. For most studied sizes good agreement is found between the experimental and the calculated ionization energies for the lowest-energy isomer. In the Si_nLi₃ (n = 5–11) series, positively charged lithium atoms surround a negatively charged silicon framework and mainly act as electron donors. Upon sequential lithium addition along the Si₈Li_m (m = 1–6) series, the Si₈ framework deforms from a rhombic (m = 0, 1) over a tetracapped tetragon (m = 1–4) to a square antiprism (m = 4–6) structure. Subsequent addition of lithium implies the addition of excess electrons to the silicon framework, which is reflected in a decrease of the ionization energy with increasing lithium content. This decrease is non-monotonous and odd–even alternations, reflecting the higher stability of clusters with an even number of electrons, are observed. In addition, post-threshold features in the experimental photoionization efficiency curves of Si_nLi₃ (n = 8–11) may be related to the computed density of states of the corresponding lowest energy isomers.