Data are presented from the field measurement campaign held in March and April 1995 at Great Dun Fell, Cumbria, Gas phase measurements were made to the west of Great Dun Fell; concentrations of nitrogen oxides (NO4 and NO2), peroxy aceiylnitrate (PAN), ozone and sulphur dioxide were made at the Fell Gate, at the foot of the hill, and gaseous ammonia was measured at the Mine Road below the summit. On the summit, measurements of concentrations of the ions H+,NH4+, NO2- and total dissolved siV in cloud water, and nitrogen oxides and sulphur dioxide were made. Pollutant gas concentrations varied considerably throughout the period, e.g, concentrations of NO, ranged from 30 ppbV to 0.1ppbV. Sulphur dioxide concentrations were generally correlated with those for NO4 but peak concentrations did not rise above 10 ppbV. Concentrations of ammonia gas were generally small, below 3 pg m-1 throughout the period. Five major periods of cloud occurred during the campaign. On-line cloud chemistry measurements for each period are described. In general the whole period was characterised by very low acidity. Cloud water ion concentration was dominated by ammonium ion probably resulting from the dissolution of gaseous ammonia. The data obtained provide evidence of the occurrence of a number of gas-liquid phase processes during the period which give an insight into the chemical processing of gases by the cap cioud on the summit.