The results of a study on the effect of oxidation on the AP index and acidity of three Nigerian coals are reported. The coals were oxidized in air over a period of 35 days at both 100 and 50°C. The heating value, slurry pH (acidity), and the ΔP index of three Nigerian coal sample were monitored as a function of oxidation time. The results revealed a decrease in ΔP index and an increase in the acidity of all three coals with increase in oxidation time. The heating value of the coals was also reduced by the oxidation.