Vertical cavity surface emitting laser diodes (VCSELs) are being investigated for use in the on-detector transmitters of the ATLAS liquid argon calorimeter optical read-out links. The radiation tolerance of optical components mounted on-detector is a critical concern. Packaged VCSEL emitters from Honeywell and Mitel have been irradiated with neutrons up to ( 2.1±0.4)×10 13 n (1 MeV equivalent in Si) cm −2 and gammas up to (5.1±0.2) kGy. The packaged components were mounted in ‘ST’ style fibre receptacles and monitored through-out the irradiation. A radiation-induced attenuation of 10–15% was seen after neutron irradiation. The induced attenuation after the gamma irradiation was < 5%. The threshold current did not change in either case. An annealing current of between 5 and 8 mA restored the light output levels to pre-irradiation levels after approximately 12 h for the neutron irradiated devices. The gamma irradiated devices exhibited no such behaviour. (Elsevier)