Three equations for estimating the concentration of free aluminium, [Al3+], from the activity of free fluoride, (F‐), were compared to assess their suitability for estimating [Al3+] in acid soil solutions and in competition studies. We then studied the ability of humic acids to compete with F for Al by comparing the behaviour of the humic acids in the presence of F and Al with that of several carboxylic acids under the same conditions. All three methods of estimating [Al ] were limited in their applicability to acid soil solutions but were uitable for estimating [Al3+] in competition studies when equimolar quantities of AlTand FTwere used. Humic acids extracted by NaOH and by Na2H2P2O7decreased the amount of Al complexed with F by 21 ‐100% at pH 4 and 6 and ratios of Cmoles of humic acid: F>l. The Al humate complex precipitated when the Al: humic acid ratio was >1. Both humic acids appeared to be more effective than simple carboxylic acids at competing with F for Al.