A campaign of pure helium discharges in the JET tokamak equipped with the MarkIIGB divertor has been performed. This paper describes some of the experimental observations of divertor detachment obtained in L-mode density ramp He discharges and presents a selection of results from the application of B2.5-Eirene code modelling to a JET He plasma. Detachment is very different from that observed in equivalent D discharges. Particle fluxes remain attached up to higher densities and, with decreasing input power, particle detachment occurs earlier in density. However, in contrast to comparable D plasmas, the low measured target T-e at all but the lowest n(e) in helium leads to an earlier decrease in power flux. Simulations show that detachment is caused by the escape of He neutrals from the target vicinity where, together with the He+ ions they radiate along the separatrix and above the X-point area, starving regions downstream of power. (C) 2003 Elsevier Science B.V. All rights reserved.