The pH, electrical conductivity and runoff response of the Allt à Mharcaidh catchment is assessed for the period 1 November 1985 to 30 April 1986. Hydrographs generated by three distinct types of input are discussed; rain, snowmelt and rain-on-snow. Each produces a distinct response at the catchment outflow and the close relationship between discharge and pH emphasises the importance of hydrological factors in determining stream acidity. Interpreting the data with a view to identifying hydrological pathways, however, presents conflicting hypotheses. The data show that moorland catchments are susceptible to severe acid pulses, the most acidic being in response to snowmelt.