The ionization bands and electron affinities of medium-size BnNn clusters (n = 3, 4, 5) have been investigated by means of one-particle Green＇s function calculations using the outer-valence Green＇s function (OVGF) approximation or the third-order algebraic diagrammatic construction [ADC(3)] scheme. Despite their structural similarity with isoelectronic cumulenic carbon chains and rings, these clusters do not exhibit a significant breakdown of the orbital picture of ionization, with the exception of the N-2s bands (epsilon(b) > 25 eV), which relate exclusively to complex sets of shake-up lines. The main (one-hole) ionization bands provide specific signatures for ring topologies based on n equivalent vertices, whereas a reduction of the curvature of N-B-N bridges with increasing system size can be followed from the N-2s bands. The B3N3- anion is slightly stable against electron loss, whereas negative vertical electron affinities are obtained for B4N4 and B5N5.