Small area (1×1 cm 2) microstrip detectors, made with a p +-n diode structure on FZ silicon substrates, both with and without oxygen enrichment, have been irradiated with 24 GeV/c protons to fluences of 1.9, 2.9 and 5.1×10 14 p/cm 2 . Their charge collection properties have been studied using a 106 Ru beta-source with a wide bandwidth current amplifier and compared with those for a non-irradiated device. The integrated charge collected at different times (10, 25, 40 and 80 ns ) has been used to estimate the effect of ballistic deficit. Predictions for the reduction in charge collection efficiency expected at fluences as high as 10 15 cm −2 are presented using a parameterization described in earlier work which also fits this data well.