Process for preparing the substituted phosphonoalkanoic acid of the formula ##STR1## wherein: R.sup.1 is alkyl of 1-8 carbon atoms, aralkyl of 7-10 carbon atoms, phenyl, or substituted phenyl of 6-10 carbon atoms;R.sup.2 is H, hydrocarbyl of 1-18 carbon atoms, or substituted hydrocarbyl of 1-18 carbon atoms, wherein in said hydrocarbyl groups the carbon atom attached to the ##STR2## moiety is substituted with two or three hydrogen atoms or is a carbon atom of an aromatic ring; andY is oxygen or sulfur,which process comprises contacting and reacting, at a temperature of about -70.degree. C. to about 125.degree. C., a pressure of about 1-100 atmospheres (0.1-10 MPa), and for a time sufficient to effect reaction, the phosphite ester of the formula ##STR3## wherein R.sup.1 and Y are as defined above, with the .alpha.-halocarboxylic acid of the formula R.sup.2 CHXCO.sub.2 H, wherein X is Cl, Br, or I and R.sup.2 is as defined above, in the presence of at least two moles of base per mole of limiting reagent, said base having a pKa greater than about 15, to produce the substituted phosphonoalkanoic acid, and process for preparing the substituted acrylic acid of the formula R.sup.3 R.sup.4 C.dbd.CR.sup.2 COOH wherein:R.sup.2 is as defined above; andR.sup.3 and R.sup.4 are the same or different and are selected from H, hydrocarbyl of 1-18 carbon atoms, and substituted hydrocarbyl of 1-18 carbon atoms or R.sup.3 and R.sup.4 taken together are cycloalkylene or substituted cycloalkylene of 4-18 carbon atoms,which process comprises:(a) carrying out the aforesaid reaction to produce the substituted phosphonoalkanoic acid except that at least three moles of base are present; and(b) contacting and reacting, at a temperature of about -70.degree. to about 125.degree. C., a pressure of about 1-100 atmospheres (0.1-10 MPa), and for a time sufficient to effect reaction, the substituted phosphonoalkanoic acid produced in step (a) with the carbonyl compound of the formula R.sup.3 R.sup.4 C.dbd.O, wherein R.sup.3 and R.sup.4 are as defined above, to produce the substituted acrylic acid.