This paper investigates the stability of dilution series of pin and nip microcrystalline silicon solar cells under low-energy proton irradiation (E = 405 keV). Variation of electrical parameters, defect-related absorption and Urbach parameter are investigated as a function of irradiation and annealing steps. Highly microcrystalline cells show a relative efficiency loss of up to 80% after proton irradiation. The efficiency loss is observed not to be completely reversible under thermal annealing. Increase of defect-related absorption and Urbach parameter is also only partially reversible. The electrical parameters (Jsc, Voc, FF) show proton-induced reductions which increase with crystallinity for both pin and nip series; short-circuit current density suffers the largest variations with relative losses of up to 65%. Defect-related absorption is shown to be low for cells of medium crystallinity, before and after irradiation. © 2006 Elsevier B.V. All rights reserved.