Abstract: Mono-alkyl acid phosphates having the formula: ##STR1## wherein R is straight or branched alkyl having from 1 to about 25 carbon atoms, are prepared by reacting alcohol with P.sub.2 O.sub.5 in the presence of tetrasodium pyrophosphate and hydrogen peroxide. The process involves initially adding P.sub.2 O.sub.5 to a mixture of alcohol and tetrasodium pyrophosphate followed by continuous or sequential addition of water and hydrogen peroxide, P.sub.2 O.sub.5 (in stoichiometic excess) and then alcohol. Additional hydrogen peroxide is added at the end of the reaction. A mixture of mono- and di-alkyl acid phsophate is obtained with a mono-content of over about 70 percent by weight.