The radical amplifier is an instrument used to measure radical concentrations in the troposphere. The critical parameter in determining the sensitivity is the chain length, which is shown to decrease with increasing water vapour in the reactor. When compared to measurements in dry air, this decrease is a factor 2 at a relative humidity of 40%. This suggests that field measurements using the radical amplifier may be underestimating the ambient radical concentration by a similar factor. One source of this deterioration in performance is an increase in the loss of radicals to the walls of the reactor, although there also appears to be a contribution from a water dependence on the gas phase chemistry. ¿ 1998 American Geophysical Union