<latex>$Sphagnum$</latex>-dominated peat bogs are naturally acid and cover perhaps 1% of the Earth's land surface. The temporal and spatial variations of pH in a peat-bog in the northern Pennines are used to illustrate the relative importance of different sources of acidity. At this site, where the pH of rain in summer was 4.0-4.2, rain and cation exchange of metal cations in the rain for H<latex>$^+$</latex> in the newly grown Sphagnum were of about equal importance, though cation exchange probably dominated on hummocks in summer. Strongly acid `events' are likely to follow heavy rain that follows a dry spell. The effects of interconversion of sulphur- and nitrogen-containing compounds on acidity is obscure.