The reactions of di-n-alkylphosphine oxides (1) with halocarboxylic acids (2) and their esters (3) were carried out in boiling sodium ethoxide and methoxide solutions, respectively. The products were identified by P%, NV, and IR and MS spectra. Di-n-hexylphosphine oxide (1b) was reacted with haloacetic and 3-halopropionic acids X- (CH2) n-CO2H, (2) (X=Cl, Br; n=1, 2) to give di-n-hexylphosphinic and 3- (di-n-hexylphosphinyl) propionic acids ((4b) and (5b)), respectively. The yield increased with increasing the concentration of sodium ethoxide. Reactions of (1) with methyl bromoacetate and 3-bromopropionate ((3a) and (3b)), followed by alkaline (potassium hydroxide) hydrolysis, gave di-n-alkylphosphinic and 3- (di-n-alkylphosphinyl) propionic acids ((4) and (5)), respectively. In the case of (3a), the optimum condition was (1) : (3a) : [NaOMe] =1 : 1 : 1 (mol/mol/mol), but the yield of (5) increased with increasing concentration of sodium methoxide in the reaction with (3b). The reaction of di-n-heptylphosphine oxide (1c) with ethyl 2-bromobutyrate (1 : 1 : 1) gave 93% of di-n-heptylphosphinic acid (4c), but a trace of (4c) was obtained with ethyl acetate. An intermediate ester, methyl 3- (di-n-hexylphosphinyl) propionate (6b), was isolated in the reaction of (1b) with (3b) (1 : 1 : 2.2, t=5h) without the hydrolysis, and was easily hydrolyzed by potassium hydroxide. For the formation of (4) which is a new oxidation reaction of (1), a mechanism involving a nucleophilic attack of the reactive species R2P (O) Na (7) on the carbonyl carbon of (2) (n=1) or (3a) to give an intermediate enol phosphate analog (9) was proposed and discussed. For the formation of (5), a possible mechanism involving a nucleophilic addition of (7) to the activated olefin produced by the ordinary alkoxide promoted 1, 2-elimination of hydrogen halide from (2) (n=2) or (3b) was proposed and discussed.